<PAGE>

    As filed with the Securities and Exchange Commission on February 8, 1999
                                                   REGISTRATION NO. 333-
- -------------------------------------------------------------------------------

                                  UNITED STATES
                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549

              -----------------------------------------------------

                                    FORM S-8
                             REGISTRATION STATEMENT
                                      UNDER
                           THE SECURITIES ACT OF 1933

              -----------------------------------------------------

                          ALEXION PHARMACEUTICALS, INC.
             (Exact Name Of Registrant As Specified In Its Charter)

                DELAWARE                           13-3648318
     (State or Other Jurisdiction of            (I.R.S. Employer
      Incorporation or Organization)           Identification No.)


                           25 SCIENCE PARK, SUITE 360
                          NEW HAVEN, CONNECTICUT 06511
                                 (203) 776-1790
                    (Address of Principal Executive Offices)

              -----------------------------------------------------

                          ALEXION PHARMACEUTICALS, INC.
                  1992 STOCK OPTION PLAN FOR OUTSIDE DIRECTORS
                            (Full Title of the Plan)

                               LEONARD BELL, M.D.
                          ALEXION PHARMACEUTICALS, INC.
                           25 SCIENCE PARK, SUITE 360
                          NEW HAVEN, CONNECTICUT 06511
                                 (203) 776-1790
                    (Name, address, including area code, and
                     telephone number of agent for service)

              -----------------------------------------------------

Copies of all communications, including all communications sent to the agent for
                          service, should be sent to:

                            MERRILL M. KRAINES, ESQ.
                           FULBRIGHT & JAWORSKI L.L.P.
                                666 FIFTH AVENUE
                            NEW YORK, NEW YORK 10103
                                 (212) 318-3261

                         CALCULATION OF REGISTRATION FEE


<TABLE>
<CAPTION>
                                                                 PROPOSED            PROPOSED        
           TITLE OF SECURITIES            AMOUNT TO BE            MAXIMUM             MAXIMUM        AMOUNT OF
            TO BE REGISTERED               REGISTERED          OFFERING PRICE        AGGREGATE      REGISTRATION
           -------------------           --------------             PER              OFFERING           FEE
                                                                  SHARE(1)           PRICE(1)       ------------
                                                               --------------       -----------      
<S>                                    <C>                   <C>                  <C>             <C>              
Common Stock, par value of $.0001 per     
share.................................    200,000 shares           $13.375           $2,675,000         $745.00
</TABLE>


(1)  The price is estimated pursuant to Rule 457(h) of the Securities Act of
     1933, as amended (the "Act"), solely for the purpose of calculating the
     registration fee and is the product resulting from 


<PAGE>

     multiplying 200,000, the number of shares registered by this Registration
     Statement as to which options may be granted under the Alexion
     Pharmaceuticals, Inc. 1992 Stock Option Plan for Outside Directors, by
     $13.375, the average of the high and low prices of Alexion Pharmaceuticals,
     Inc. Common Stock as reported on The Nasdaq National Market on February 2,
     1999.

- -------------------------------------------------------------------------------


<PAGE>

                                     PART I

              INFORMATION REQUIRED IN THE SECTION 10(A) PROSPECTUS

     In accordance with the rules and regulations of the Securities and Exchange
Commission, the documents containing the information called for in Part I of
Form S-8 will be sent or given to individuals who participate in Alexion
Pharmaceuticals, Inc. 1992 Stock Option Plan for Outside Directors and are not
being filed with or included in this Form S-8.


                              PROSPECTUS STATEMENT

The material which follows, up to but not including the page beginning Part II
of this Registration Statement, constitutes a prospectus, prepared in accordance
with the requirements of Part I of Form S-3 pursuant to General Instruction C to
Form S-8, to be used in connection with resales of securities acquired under the
Alexion Pharmaceuticals, Inc. 1992 Stock Option Plan for Outside Directors by
certain directors qualifying under such employee benefit plan, as defined in
Rule 405 under the Securities Act of 1933, as amended.



<PAGE>


                  SUBJECT TO COMPLETION, DATED FEBRUARY 8, 1999

PROSPECTUS
                         200,000 SHARES OF COMMON STOCK

                          ALEXION PHARMACEUTICALS, INC.
                           25 SCIENCE PARK, SUITE 360
                               NEW HAVEN, CT 06511
                                 (203) 776-1790

     This prospectus relates to the offer and sale of up to 200,000 shares of
our common stock by certain selling stockholders. These selling stockholders are
directors who have acquired or may acquire these shares upon the exercise of
stock options. The stock options were or will be granted pursuant to Alexion
Pharmaceuticals, Inc. 1992 Stock Option Plan for Outside Directors. On 
February 5, 1999, the closing price of the Common Stock was $13 7/8 per share.

     The selling stockholders may offer their shares from time to time, in
different types of transactions, including brokerage and negotiated transactions
or otherwise, at market prices prevailing at the time of sale or at negotiated
or other prices.

     We will receive no proceeds from any of these sales although we will
receive the exercise prices of the stock options.


                NASDAQ NATIONAL MARKET(SM) TRADING SYMBOL - ALXN


                               ------------------


     THIS INVESTMENT INVOLVES A HIGH DEGREE OF RISK. YOU SHOULD PURCHASE SHARES
ONLY IF YOU CAN AFFORD A COMPLETE LOSS. SEE "RISK FACTORS" BEGINNING ON PAGE 10.


                               ------------------


     NEITHER THE SECURITIES AND EXCHANGE COMMISSION NOR ANY STATE SECURITIES
COMMISSION HAS APPROVED OR DISAPPROVED OF THESE SECURITIES OR DETERMINED IF THIS
PROSPECTUS IS TRUTHFUL OR COMPLETE. ANY REPRESENTATION TO THE CONTRARY IS A
CRIMINAL OFFENSE.


                THE DATE OF THIS PROSPECTUS IS FEBRUARY 8, 1999.


     THE INFORMATION IN THIS PROSPECTUS IS NOT COMPLETE AND MAY BE CHANGED. THE
SELLING STOCKHOLDERS MAY NOT SELL THESE SECURITIES UNTIL THE REGISTRATION
STATEMENT FILED WITH THE SECURITIES AND EXCHANGE COMMISSION IS EFFECTIVE. THIS
PROSPECTUS IS NOT AN OFFER TO SELL THESE SECURITIES AND IT IS NOT SOLICITING AN
OFFER TO BUY THESE SECURITIES IN ANY STATE WHERE THE OFFER OR SALE IS NOT
PERMITTED.


<PAGE>

                                TABLE OF CONTENTS

<TABLE>
<CAPTION>

                                                                                                                Page
                                                                                                                ----
<S>                                                                                                             <C>
Where You Can Find More Information...............................................................................2

Cautionary Statement Regarding Forward-Looking Statements.........................................................3

Prospectus Summary................................................................................................4
         Our Business.............................................................................................4
         Additional Information...................................................................................9

Risk Factors.....................................................................................................10
         Operating Losses; Uncertainty of Future Profitability...................................................10
         Early State of Product Development; Risks of Clinical Trials............................................10
         Need for Additional Funds...............................................................................11
         Year 2000 Compliance....................................................................................12
         Rapid Technological Change..............................................................................12
         Patent, License and Proprietary Rights Uncertainties....................................................12
         No Assurance of Food & Drug Administration Approval; Government Regulation..............................13
         No Currently Approved Xenotransplantation-Based Products................................................14
         Substantial Competition.................................................................................14
         Dependence on Qualified Personnel.......................................................................15
         Dependence on Outside Parties and Collaborators.........................................................15
         Limited Manufacturing, Marketing, Sales, Clinical Testing and Regulatory Compliance
                  Capability.....................................................................................16
         Uncertainty of Availability of Healthcare Reimbursement.................................................16
         Product Liability; Potential Liability for Human Clinical Trials........................................16
         Volatility of Share Price...............................................................................17
         Dilutive Effect of Stock Issuances, Grants, Options and Warrants........................................17
         Possible Adverse Impact on Holders of Common Stock; Anti-takeover Provisions; Rights
                  Plan...........................................................................................17
         Ownership by Management and Principal Stockholders......................................................18
Use of Proceeds..................................................................................................18
Selling Stockholders.............................................................................................19
Plan of Distribution.............................................................................................20
Legal Matters....................................................................................................21
Experts..........................................................................................................21
</TABLE>


                                       -1-


<PAGE>

                       WHERE YOU CAN FIND MORE INFORMATION

     We file annual, quarterly and special reports, proxy statements and other
information with the Securities and Exchange Commission. You may read and copy
any reports, statements or other information filed by us at the Commission's
public reference room at Room 1024, Judiciary Plaza, 450 Fifth Street, N.W.,
Washington, D.C. 20549 and the regional offices of the Commission located at
Seven World Trade Center, 13th Floor, New York, New York 10048, and 500 West
Madison Street, Chicago, Illinois 60661. Copies of such material can be also
obtained from the Public Reference Section of the Commission at 450 Fifth
Street, N.W., Washington, D.C. 20549, and its public reference rooms in New
York, New York and Chicago, Illinois, at prescribed rates. Please call the
Commission at 1-800-SEC-0330 for further information on the public reference
rooms. Copies of such information may also be inspected at the reading room of
the library of the National Association of Securities Dealers, Inc., 1735 K
Street, N.W., Washington, D.C. 20006. Our filings with the Commission are also
available to the public from commercial document retrieval services and at the
Commission's web site at "http://www.sec.gov."

     We are allowed to "incorporate by reference" the information we file with
the Commission (File No. 0-27756), which means that we can disclose important
information to you by referring you to another document we filed with the
Commission. The information incorporated by reference is an important part of
this Prospectus, and information that we file later with the Commission will
automatically update and supersede this information. We incorporate by reference
the documents listed below and any future filings made with the Commission under
Sections 13(a), 13(c), 14 or 15(d) of the Securities Exchange Act of 1934, as
amended, until the selling stockholders sell all of the shares of common stock:

          (a)  Our Annual Report on Form 10-K for the fiscal year ended July 31,
               1998;

          (b)  Our Quarterly Report on Form 10-Q for the quarterly period ended
               October 31, 1998;

          (c)  Our Current Reports on Form 8-K, filed on October 9, 1998,
               December 31, 1998 and January 29, 1999; and

          (d)  Our Registration Statement on Form 8-A, dated February 12, 1996.

     You should read the information relating to us in this Prospectus together
with the information in the documents incorporated by reference.

     Any statement contained in a document incorporated by reference herein,
unless otherwise indicated therein, speaks as of the date of the document.
Statements contained in this Prospectus may modify or replace statements
contained in the documents incorporated by reference.

     We will furnish without charge to you, upon request, a copy of any or all
of the documents described above, except for exhibits to such documents, unless
such exhibits are specifically incorporated by reference into such documents.
Requests should be addressed to: Alexion Pharmaceuticals, Inc., 25 Science Park,
Suite 360, New Haven, Connecticut 06511, (203) 776-1790, Attention: David W.
Keiser, Executive Vice President and Chief Operating Officer. We furnish our
stockholders with an annual report containing audited financial statements. In
addition, we may furnish such other reports as may be authorized, from time to
time, by the Board of Directors.

                                       -2-


<PAGE>

     This prospectus is part of a registration statement we filed with the
Commission. You should rely only on the information incorporated by reference or
provided in this prospectus or any supplement. We have not authorized anyone
else to provide you with different information. The selling stockholders will
not make an offer of the shares of common stock in any state where the offer is
not permitted. You should not assume that the information in this prospectus or
any supplement is accurate as of any date other than the date on the front of
those documents.

            CAUTIONARY STATEMENT REGARDING FORWARD-LOOKING STATEMENTS

     This prospectus contains certain "forward-looking statements" as defined in
the Private Securities Litigation Reform Act of 1995 and information relating to
us that are based on the beliefs of our management, as well as assumptions made
by and information currently available to our management. When used in this
prospectus, the words "estimate," "project," "believe," "anticipate," "intend,"
"expect" and similar expressions are intended to identify forward-looking
statements. These forward-looking statements reflect our current views with
respect to future events and are subject to risks and uncertainties that could
cause actual results to differ materially from those contemplated in these
forward-looking statements, including those risks discussed under "Risk
Factors." You are cautioned not to place undue reliance on these forward-looking
statements, which speak only as of the date on this prospectus. We have no
obligation to publicly release any revisions to these forward-looking statements
to reflect events or circumstances after the date of this prospectus or to
reflect the occurrence of unanticipated events.

                                       -3-


<PAGE>

                               PROSPECTUS SUMMARY

     THIS SUMMARY HIGHLIGHTS SELECTED INFORMATION FROM THIS PROSPECTUS AND MAY
NOT CONTAIN ALL OF THE INFORMATION THAT IS IMPORTANT TO YOU. TO UNDERSTAND THE
TERMS OF OUR SECURITIES, YOU SHOULD CAREFULLY READ THIS DOCUMENT. YOU SHOULD
ALSO READ THE DOCUMENTS WE HAVE REFERRED YOU TO IN THE SECTION ENTITLED "WHERE
YOU CAN FIND MORE INFORMATION" ON PAGE 1 FOR INFORMATION ON OUR COMPANY AND OUR
FINANCIAL STATEMENTS.

                                  OUR BUSINESS

     We are a biopharmaceutical company engaged in the research and development
of proprietary immunoregulatory compounds for the treatment of autoimmune and
cardiovascular diseases. We are developing C5 complement inhibitors and apogens,
two classes of potential therapeutic compounds designed to selectively target
specific disease-causing segments of the immune system. We believe that our C5
inhibitors and apogens, which are based upon distinct immunoregulatory
technologies, may have the advantage of achieving a higher level of efficacy
with the potential for reduced side effects when compared to existing
therapeutic approaches. For the longer term, as an outgrowth of our core
technologies, we are developing non- human cell tissue and our Unigraft organ
products which are designed for transplantation into humans, or
xenotransplantation, without clinical rejection.

     KEY DISEASE TARGETS FOR OUR C5 INHIBITOR PROGRAM

     -    Acute coronary syndromes, including cardiopulmonary bypass, acute
          myocardial infarction, coronary angioplasty and unstable angina
     -    Autoimmune disorders, including systemic lupus and rheumatoid
          arthritis

     KEY DISEASE TARGETS FOR OUR APOGEN PROGRAM

     -    Autoimmune disorders, including multiple sclerosis and diabetes
          mellitus

     KEY  DISEASE TARGETS FOR OUR UNIGRAFT PROGRAM

     -    Spinal cord injury
     -    Parkinson's disease
     -    Organ failure

     Currently, we are conducting human clinical trials in cardiopulmonary
bypass, rheumatoid arthritis and systemic lupus patients. We need to undertake
and complete further tests in order to confirm our beliefs. There can be no
assurance as to the results of any of these tests.

OUR DRUG DEVELOPMENT STRATEGY

     Our strategy is to develop novel immunoregulatory therapeutics for disease
states, disorders and clinical indications for which we believe treatment
options are either non-existent or inadequate.

     Currently available therapies for certain autoimmune, cardiovascular and
neurologic diseases, in which the immune system attacks the patient's own
tissue, broadly suppress the entire immune system, thus causing potentially
severe side effects. In contrast, our proprietary compounds are designed to be
more 

                                      -4-

<PAGE>

effective with reduced side effects when compared to currently available
therapies by generally targeting only the specific disease-causing segments of
the immune system, leaving the remaining segments of the immune system intact to
perform their normal protective functions.

INTRODUCTION TO THE HUMAN IMMUNE SYSTEM

     The role of the human immune system is to defend the body from attack or
invasion by infectious agents or pathogens. This is accomplished through a
complex system of proteins and cells, primarily complement proteins, antibodies
and various types of white blood cells, each with a specialized function. Under
normal circumstances, complement proteins, together with antibodies and white
blood cells, act beneficially to protect the body by removing pathogenic
microorganisms, cells containing antigens (foreign proteins) and disease-causing
immune complexes (combinations of antigens and antibodies). However, any number
of stimuli (including antibodies, pathogenic microorganisms, injured tissue,
normal tissue, proteases (inflamma tory enzymes) and artificial surfaces) can
locally activate complement proteins in a cascade of enzymatic and biochemical
reactions (the "complement cascade") to form inflammatory byproducts. In the
case of cardio vascular disorders such as myocardial infarction (death of heart
tissue), this may lead to additional significant damage to the heart tissue,
and, in the case of rheumatoid arthritis, this may lead to severe joint
inflammation. T-cells, a type of white blood cell, play a critical role in the
normal immune response by recognizing cells containing antigens, initiating the
immune response, attacking the antigen-containing tissue and directing the
production of antibodies targeting the antigens, resulting in the elimination of
the antigen-bearing foreign organism. When a T-cell mistakenly attacks host
tissue, the T-cell may cause an inflammatory response resulting in tissue
destruction and severe autoimmune disease. In the case of multiple sclerosis,
this may cause severe and crippling destruction of nerve fibers in the brain.

C5 COMPLEMENT INHIBITOR IMMUNOTHERAPEUTICS

     We are developing specific and potent biopharmaceutical C5 inhibitors which
are designed to intervene in the complement cascade at what we believe to be the
optimal point so that the disease-causing actions of complement proteins
generally are inhibited while the normal disease-preventing functions of
complement proteins generally remain intact. In laboratory and animal models of
human disease, we have shown that our C5 inhibitors are effective in
substantially preventing inflammation during cardiopulmonary bypass, reducing
tissue damage during myocardial infarction, reducing the incidence and severity
of inflammation and joint damage in rheumatoid arthritis, enhancing survival in
lupus, and preserving kidney function in nephritis or kidney inflammation.

     We are currently developing two C5 inhibitors:

     -    5G1.1-SC: a short-acting, compatible for human use, single chain
          antibody for treating acute coronary syndromes, including
          cardiopulmonary bypass procedures and myocardial infarction; and

     -    5G1.1: a long-acting, compatible for human use, monoclonal antibody
          for treating chronic disorders such as lupus and rheumatoid arthritis.

                                       -5-


<PAGE>

SUMMARY OF C5 INHIBITOR PRODUCT DEVELOPMENT

COLLABORATION AGREEMENT WITH PROCTER & GAMBLE PHARMACEUTICALS

     In January 1999, we entered into a collaboration agreement with Procter &
Gamble Pharmaceuticals, a part of Procter & Gamble Company, to develop and 
commercialize 5G1.1-SC. Under the terms of the agreement, together with 
Procter & Gamble, we will assess 5G1.1-SC for use in coronary artery bypass 
graft surgery, angioplasty and other acute cardiovascular problems, such as 
myocardial infarction and unstable angina, as well as other applications. The 
collaboration includes the potential for us to receive up to $95 million in 
payments, which will include an upfront license fee, milestone payments and 
research and development support payments. While we and Procter & Gamble will 
jointly design and implement clinical development, by terms of the agreement, 
Procter & Gamble is to pay for all of the costs of clinical development and 
manufacturing. We will also receive royalties if and when products are sold. 
We have also retained U.S. co-promotion rights and worldwide manufacturing 
rights for the drug. Procter & Gamble received U.S. co-promotion rights, as 
well as marketing rights outside of the United States. Procter & Gamble does 
not have rights to any other product we are developing.

CARDIOPULMONARY BYPASS

     In March 1996, we filed an investigational new drug application with the
U.S. Food and Drug Administration for 5G1.1-SC, our lead anti-inflammatory
complement inhibitor drug candidate. After receiving FDA authorization, we began
a Phase I clinical trial in healthy male volunteers in June 1996. Results of the
Phase I trial indicated that a single dose administration of 5G1.1-SC was safe
and well-tolerated in the study population. In September 1996, we received FDA
authorization for our second clinical trial. In October 1996, we commenced a
Phase I/II study of 5G1.1-SC in patients undergoing cardiopulmonary bypass.

     In July 1997, we released preliminary results from this Phase I/II clinical
study of 17 patients undergoing cardiopulmonary bypass. Treatment with 5G1.1-SC
reduced the more than ten-fold increase in the level of activated complement
byproducts experienced by patients on placebo during cardiopulmonary bypass in a
dose-dependent manner. With FDA approval, we initiated a Phase IIa
cardiopulmonary bypass clinical study including an additional 18 patients. In
October 1997, additional preliminary results indicated that 5G1.1-SC
significantly reduced leukocyte activation (inflammation) as compared to
placebo. In April 1998, we announced clinical results from the Phase I/II and
Phase IIa cardiopulmonary bypass studies which indicated that 5G1.1-SC
significantly reduced cardiac damage, new cognitive (brain) deficits and blood
loss in patients undergoing coronary artery bypass graft surgery with
cardiopulmonary bypass.

     In December 1998, we commenced dosing patients undergoing coronary artery
bypass graft surgery with or without accompanying valve surgery during 
cardiopulmonary bypass in a Phase IIb clinical trial with 5G1.1-SC. This 
study is designed to gather clinical data to augment and extend previous 
findings regarding the safety profile and pharmacokinetics of 5G1.1-SC and 
its efficacy in reducing the life-threatening inflammatory complications that 
can be triggered by cardiopulmonary bypass procedures.

ACUTE MYOCARDIAL INFARCTION

                                       -6-


<PAGE>

     In November 1998, we commenced dosing subjects in a Phase I clinical trial
that was designed to evaluate dosing regimens for subsequent cardiopulmonary
bypass and myocardial infarction clinical trials. Later that month, we announced
the successful completion of the first stage of this trial and selection of a
dosing regimen for the subsequent cardiopulmonary bypass trial. The results of
this trial indicated that 5G1.1- SC was well tolerated at doses more than three
times as high as had been previously administered.

     As a result of positive results in preclinical studies, we plan to file in
1999 an investigational new drug application using 5G1.1-SC for the treatment of
acute myocardial infarction or heart attack.

RHEUMATOID ARTHRITIS

     In December 1997, we filed an investigational new drug application with the
FDA for 5G1.1 in the treatment of rheumatoid arthritis in patients. After
receiving FDA authorization, we began a Phase I/II multi- center clinical trial
in rheumatoid arthritis patients in July 1998. We completed patient dosing in
December 1998.

LUPUS NEPHRITIS

     In late December 1997, we filed an investigational new drug application
with the FDA for 5G1.1 in a clinical indication for the treatment of patients
suffering from Systemic Lupus Erythematosus. After receiving FDA authorization,
we began a Phase I/II clinical trial in lupus patients in July 1998. We
completed patient dosing in January 1999.

APOGEN IMMUNOTHERAPEUTICS

     Our apogen compounds are based upon discoveries at the National Institutes
of Health, which are exclusively licensed to Alexion, and upon further
discoveries by Alexion. The highly specific recombinant apogens under
development are designed to selectively eliminate disease-causing T-cells in
patients with certain autoimmune diseases, including multiple sclerosis and
diabetes mellitus. We have demonstrated that our lead proprietary apogen, MP4,
is effective at preventing neurologic disease in animal models of multiple
sclerosis.

SUMMARY OF APOGEN PRODUCT DEVELOPMENT

MULTIPLE SCLEROSIS

     In February 1998, we filed an investigational new drug application with the
FDA for MP4 for the treatment of patients suffering from multiple sclerosis.
After completion of additional preclinical studies and amendment of the clinical
protocol in line with the preferred route of administration, we expect to
initiate a Phase I/II clinical trial in multiple sclerosis patients.

DIABETES MELLITUS

     We are currently developing apogen DM which is designed to prevent and
treat insulin-dependent diabetes mellitus by eliminating antigen-specific
T-cells which are responsible for the pancreatic B-cell destruction. We have
established animal models of diabetes and have commenced initial preclinical
studies with apogen DM prototypes. In June 1998, at the American Diabetes
Association's 58th Annual Scientific 


                                      -7-

<PAGE>

Sessions, we presented preclinical data indicating that a new drug candidate,
IG2, was highly effective in suppressing the development of insulin-dependent
diabetes mellitus in two different animal models.

THE UNIGRAFT PROGRAM

ORGAN AND TISSUE TRANSPLANTATION

     Building upon our core technologies, we are developing non-human cell
tissue and organ UniGraft products which are designed for transplantation into
humans (xenotransplantation) without clinical rejection.

     Rejection of non-human tissue by patients is generally believed to occur in
two stages:

     -    Hyperacute phase, which is very rapid, extending from minutes to
          hours; and
     -    Acute phase, which is somewhat less rapid, extending from days to
          months.

     We are designing UniGraft products to resist both
complement/antibody-mediated hyperacute rejection and T-cell-mediated acute
rejection. We have commenced studies employing the UniGraft technologies during
preclinical transplantation of genetically engineered and proprietary porcine
cells and organs. Currently, pigs are a preferred source of organ supply because
the anatomy, size, and physiology of their hearts and other organs are similar
to human organs. We have genetically engineered porcine cells that are resistant
to lysis (break-up) and activation by human complement proteins. We have also
discovered and designed porcine- specific antibodies which have been
demonstrated to selectively and significantly block the human T-cell response to
porcine tissue in IN VITRO studies. We are currently using our immunoregulatory
and molecular engineering technologies in order to develop UniGraft cells to
treat Parkinson's disease and injuries to the spinal cord, hearts, lungs,
livers, pancreases and kidneys.

     We have been focusing some of our efforts in this program at developing an
effective treatment for patients with spinal cord injury. Collaborating
scientists presented data at the 28th Meeting of the Society for Neuroscience in
November 1998 describing our novel approaches to the transplantation of pig
cells which may have implications for the treatment of spinal cord injury
patients. The report of the preclinical study included data showing that our
transgenic pig cells form a sheath around damaged neurons in animals whose
spinal cords were surgically severed. The data also showed that the animal
spinal cords that had received the pig cell transplants showed restoration of
normal nerve signal conduction.

     We have also been attempting to develop effective treatments for patients
with Parkinson's Disease. Collaborating scientists presented data at the 28th
Meeting of the Society for Neuroscience in November 1998 describing our novel
approaches to the transplantation of our transgenic pig brain cells which have
implications for the treatment of patients with Parkinson's, Alzheimer's or
Huntington's Diseases. The report of the preclinical study included data showing
restoration of brain function obtained following the first xenotransplantation
of transgenic pig nerve cells (neurons) in an animal model of Parkinson's
Disease.

     We began developing our Unigraft products in collaboration with United
States Surgical Corporation. During 1998, Tyco International, Inc. acquired
United States Surgical. In December 1998, we signed a letter of intent with Tyco
to reacquire the rights to all aspects of our xenotransplantation program that
had been obtained by Tyco when Tyco acquired United States Surgical. We are
currently in the process of completing definitive agreements in this regard.

                                      -8-

<PAGE>

                             ADDITIONAL INFORMATION

     We were founded in New Haven, Connecticut in January 1992 with scientific
founders largely drawn from the faculty of Yale University. We incorporated in
Delaware in 1992. Our principal executive offices are at 25 Science Park, New
Haven, Connecticut 06511, and our telephone number is (203) 776-1790.



                                       -9-


<PAGE>

                                  RISK FACTORS

     IN ADDITION TO THE OTHER INFORMATION IN THIS PROSPECTUS, THE FOLLOWING
FACTORS SHOULD BE CONSIDERED CAREFULLY IN EVALUATING AN INVESTMENT IN THE SHARES
OF COMMON STOCK OFFERED BY THIS PROSPECTUS.

     OPERATING LOSSES; UNCERTAINTY OF FUTURE PROFITABILITY. We have generated no
revenues from product sales. We depend on our research and development
contracts, external financing, other contracts and grants to the extent that
they can be obtained and interest income to pursue our intended business
activities. We have incurred losses since inception. We have an accumulated
deficit of approximately $44.2 million through October 31, 1998. Losses have
resulted principally from research costs in identifying and developing new
products and from general and administrative costs. We expect to incur
substantial additional operating losses over the next several years and expect
losses to increase as our research and development efforts expand and clinical
trials continue and potentially expand. Our ability to achieve profitability
depends on many factors, including:

     -    obtaining and maintaining patent protection and regulatory approval
          for our products;
     -    obtaining licenses from third parties to use technology which we may
          need ;
     -    entering into agreements for product development and commercialization
          with corporate partners; and
     -    developing the capacity to manufacture and sell products.

We may not:

     -    successfully develop, commercialize, manufacture or market any of our
          potential products,
     -    obtain required regulatory approvals, patents or third party licenses
          to technology or
     -    ever achieve profitability.

         EARLY STATE OF PRODUCT DEVELOPMENT; RISKS OF CLINICAL TRIALS. Our
research and development programs are still at an early stage with only certain
of our potential products undergoing clinical trials. Our drug discovery efforts
may not result in the development of commercially successful therapeutic drugs.
Any potential products we identify will require:

     -    significant additional development;
     -    preclinical and clinical testing;
     -    regulatory approval; and
     -    additional investment prior to their commercialization.

Each stage of product development may never be achieved. Potential products may:

     -    be ineffective or cause harmful side effects or unexpected results
          during preclinical testing or clinical trials;
     -    fail to receive necessary regulatory approvals;
     -    be difficult to manufacture on a large scale;
     -    fail to achieve market acceptance; and/or
     -    be uneconomical or be precluded from commercialization by proprietary
          rights of third parties.

                                      -10-

<PAGE>

The results from preclinical studies and early clinical trials may not be
predictive of results that will be obtained in large-scale clinical trials and
do not necessarily predict or prove safety or efficacy in humans.

     Further, clinical trials of our product candidates may not demonstrate
sufficient safety and efficacy to obtain the requisite regulatory approvals or
may not result in marketable products. Clinical trials are often conducted with
patients that are critically ill. During the course of treatment, these patients
can die or suffer other adverse medical effects for reasons that may not be
related to the pharmaceutical agent being tested but which can nevertheless
affect clinical trial results. A number of companies in the pharmaceutical
industry have suffered significant setbacks in advanced clinical trials, even
after promising results in earlier trials. Any setback could have a material
adverse effect on our business, financial condition and results of operations.
The completion of clinical trials of our product candidates may be delayed by
many factors. We cannot assure you that delays or terminations will not occur.
One factor is the rate of enrollment of patients, which generally varies
throughout the course of a clinical trial and which depends on:

     -    the size of the patient population and the number of clinical trial
          sites;
     -    the proximity of patients to clinical trial sites;
     -    the performance of the clinical trial sites; and
     -    the eligibility criteria for the clinical trial and the existence of
          competing clinical trials.

We cannot control the rate at which patients present themselves for enrollment.
We cannot assure you that the rate of patient enrollment will be consistent with
our expectations or be sufficient to enable clinical trials of our product
candidates to be completed in a timely manner. Further, we cannot be certain
that clinical trial materials will be produced in a timely manner, if at all.

     NEED FOR ADDITIONAL FUNDS. We will require substantial additional funds
          for:

     -    for conducting clinical trials;
     -    our research and product development programs;
     -    for operating expenses;
     -    for pursuing regulatory approval; and
     -    for developing required production, sales and marketing capabilities.

Except for a term loan facility for $1.2 million from a commercial bank for the
financing of certain capital expenditures and a collaboration agreement with
Procter and Gamble, we do not currently have any commitments or arrangements to
obtain any additional funds. Funds for these purposes, whether through
additional sales of securities or collaborative or other arrangements with
corporate partners or from other sources, may not be available to us when needed
or on terms favorable to us. The unavailability of additional financing could
require us to delay, scale back or eliminate some or all of our research and
product development programs or to license third parties to commercialize
products or technologies that we would otherwise undertake itself. Any of these
actions would have a material adverse effect on us. We believe that our existing
available resources, together with interest income, should be sufficient to fund
our operating expenses and capital requirements as currently planned through the
next seventeen months. However, our cash requirements may vary materially from
those now planned because of:

     -    results of research and development;
     -    results of product testing;
     -    developments in large-scale production;

                                      -11-

<PAGE>

     -    relationships with strategic partners;
     -    changes in the focus and direction of our research and development
          programs;
     -    competitive and technological factors;
     -    developments in the regulatory process; and/or
     -    other factors which are difficult to predict.

     If we are the recipient of awards from government agencies to fund research
and development, the actual timing and receipt of funding for those awards may
depend on periodically approved government budgets and appropriations, which are
outside our control. Any change in governmental needs and priorities or any
delay in legislative or other governmental action with respect to budgets and
appropriations could materially and adversely effect us.

     YEAR 2000 COMPLIANCE. The "Year 2000" issue affects computer and
information technology systems, as well as non-information technology systems
which include embedded technology such as micro-processors and micro-controllers
(or micro-chips) with date-sensitive programs that may not properly recognize
the year 2000. Systems that do not properly recognize this information could
generate inaccurate data or cause a system to fail, resulting in business
interruption. Currently, we are developing a plan and taking steps to provide
measured assurances that our computer and information technology systems and
non-information technology systems (including embedded systems such as heating,
ventilation and air conditioning systems and other analytical instruments and
equipment) are or will be Year 2000 compliant. We are also taking steps to
confirm that those third parties with material relationships with us are or will
be Year 2000 compliant. However, we cannot assure you that all material systems
will be Year 2000 compliant in a timely manner, if at all.

     RAPID TECHNOLOGICAL CHANGE. We are engaged in pharmaceutical fields
characterized by extensive research efforts, rapidly evolving technology and
intense competition from numerous organizations, including pharmaceutical
companies, biotechnology firms, academic institutions and others. New
developments are expected to continue at a rapid pace in both industry and
academia. Research and discoveries by others may render any of our programs or
potential products obsolete or uneconomical. In order to compete successfully,
we will need to complete development and obtain regulatory approval of products
that keep pace with technological developments on a timely basis. Any failure by
us to anticipate or respond adequately to techno logical developments could have
a material adverse effect on our business, financial condition and results of
operations.

     PATENT, LICENSE AND PROPRIETARY RIGHTS UNCERTAINTIES. Our success will
depend in part on our ability to:

     -    obtain and maintain United States and foreign patent protection for
          our products;
     -    preserve our trade secrets and proprietary rights; and
     -    operate without infringing on the proprietary rights of third parties
          or having third parties circumvent our rights.

Because of the length of time and expense associated with bringing new products
through development and regulatory approval to the marketplace, the healthcare
industry has traditionally placed considerable importance on obtaining patent
and trade secret protection for significant new technologies, products and
processes. We cannot assure you that any patents will issue from any of the
patent applications owned by or licensed to us. Further, we cannot assure you
that, even if patents were to issue, they will provide us with significant
protection against competitive products or otherwise be commercially valuable.
In addition, patent law relating to certain of our fields of interest,
particularly as to the scope of claims in issued patents, is still 

                                      -12-

<PAGE>

developing and it is unclear how this uncertainty will affect our patent rights.
Litigation, which could be costly and time consuming, may be necessary to
enforce patents issued to us and/or to determine the scope and validity of
others' proprietary rights, in either case in judicial or administrative
proceedings. Our competitive position also depends on unpatented trade secrets
which generally are difficult to protect. We cannot assure you that others will
not independently develop substantially equivalent proprietary information and
techniques or otherwise gain access to our trade secrets, that our trade secrets
will not be disclosed or that we can effectively protect our rights to
unpatented trade secrets. As the biotechnology industry expands and more patents
are issued, the risk increases that our potential products may give rise to
lawsuits that they infringe the patents of others. Any such lawsuits would be
costly and time consuming to us.

     We are aware of broad patents owned by third parties relating to the
manufacture, use and sale of recombinant humanized antibodies, recombinant
humanized single chain antibodies and genetically engineered animals. We have
received notice from certain of these parties regarding the existence of certain
of these patents which the owners claim may be relevant to the development and
commercialization of certain of our proposed products. With respect to certain
of these patents, which we believe are relevant for the expeditious development
and commercialization of certain of our products as currently contemplated, we
have acquired licenses. With regard to certain other patents, we have either
determined in our judgment that our products do not infringe the patents or have
identified and are testing various approaches which we believe should not
infringe the patents and which should permit commercialization of our products.
However, the owner of these patents might still seek to enforce the patent
against our so-modified commercial products or against the development
activities related to the non-modified products. If it becomes necessary, we may
not be able to obtain a license on commercially reasonable terms. If we do not
obtain necessary licenses, we could encounter delays in product market
introductions while we attempt to design around such patent or could find that
the development, manufacture or sale of products requiring such a license could
be foreclosed. Further, we cannot assure you that owners of patents that we do
not believe are relevant to our product development and commercialization will
not seek to enforce their patents against us. Such action could result in
litigation which would be costly and time consuming. We cannot assure you that
we would be successful in such litigations. We are currently unaware of any
threatened action.

     Certain licenses by which we obtained rights in and to certain technologies
require us to diligently commercialize or attempt to commercialize those
technologies. We may not meet such requirements, and failing to do so for a
particular technology could result in losing our rights to that technology.

     Currently, we have not sought to register our potential trademarks, and we
cannot assure you that we will be able to obtain registration for those
trademarks.

     NO ASSURANCE OF FOOD & DRUG ADMINISTRATION APPROVAL; GOVERNMENT REGULATION.
The preclinical and clinical testing, manufacturing and marketing of our
products are subject to extensive regulation by numerous government authorities
in the United States and other countries, including the FDA. Among other
requirements, FDA approval of our products, including a review of the
manufacturing processes and facilities used to produce such products, will be
required before our products may be marketed in the United States. Similarly,
marketing approval by a foreign governmental authority is typically required
before products may be marketed in a particular foreign country. In order to
obtain FDA approval of a product, we must, among other things, demonstrate to
the satisfaction of the FDA that the product is safe and effective for its
intended uses and that we are capable of manufacturing the product with
procedures that conform to the FDA's then current good manufacturing practice
regulations. The process of seeking FDA approvals can be costly, time consuming
and subject to unanticipated and significant delays. Approvals may not be
granted on a timely basis, or at all. Any delay in obtaining or any failure to
obtain approvals would adversely affect our ability to introduce and market
products and to generate product revenue.

                                      -13-


<PAGE>

     Our research and development processes involve the controlled use of
hazardous materials. We are subject to federal, state and local laws and
regulations governing the use, manufacture, storage, handling and disposing of
such materials and certain waste products. In the event of such an accident, we
could be held liable for any damages that result and any resulting liability
could exceed our resources. We may be required to incur significant costs to
comply with the environmental laws and regulations in the future. Our business,
financial condition and results of operations may be materially adversely
affected by current or future environmental laws or regulations.

     NO CURRENTLY APPROVED XENOTRANSPLANTATION-BASED PRODUCTS. Building upon our
core technologies, we are developing non-human organ and cell products designed
for transplantation into humans. Our approach involves xenotransplantation - the
transplantation or use of live organs, tissue and cells from one species into
another. Xenotransplantation technology is an emerging technology with, as yet,
limited clinical applications. We cannot assure you that our organ, tissue and
cell transplantation technology will result in the development of any
therapeutic products. Although several companies are focusing on
xenotransplantation-based products, this area represents a novel therapeutic
approach that has not yet been subject to extensive clinical testing.

     Xenotransplantation also poses a risk that viruses or other animal
pathogens may be unintentionally transmitted not only to a human patient
recipient, but other human beings. Recent scientific publications by others
demonstrate, under laboratory conditions, that porcine endogenous retro viruses
have the potential to infect human cells. While these viruses has not been shown
to cause any disease in pigs or humans, it is not known what effect, if any,
such viruses may have on humans. Our porcine organ, tissue and cell product
development programs would be negatively impacted by the detection of these
viruses in porcine cells in our preclinical or clinical development program or
at other companies focusing in this area.

     No xenotransplantation-based therapeutic product has been approved for sale
by the FDA. The FDA has not yet established definitive regulatory guidelines for
xenotransplantation but has proposed interim guidelines in an attempt to reduce
the risk of contamination of transplanted organ and cellular products with
infectious agents. Definitive guidelines may not be issued. We may not be able
to comply with any final definitive guidelines that may be issued. Furthermore,
our products may not be approved by the FDA or regulatory authorities in other
countries in a timely manner, if at all. Xenotransplantation-based products,
including products developed by us, may not be accepted by the medical community
or third-party payors. If accepted, the degree of acceptance may limit the size
of the market for our products.

     SUBSTANTIAL COMPETITION. The pharmaceutical and biotechnology industries
are intensely competitive. Many companies, including major pharmaceutical and
chemical companies as well as specialized biotechnology companies, are engaged
in activities similar to our activities. Certain of these companies may have:

     -    substantially greater financial and other resources;
     -    larger research and development staffs; and/or
     -    more extensive marketing and manufacturing organizations.

Many of these companies have significant experience in preclinical testing,
human clinical trials, product manufacturing, marketing and distribution and
other regulatory approval procedures. In addition, colleges, universities,
governmental agencies and other public and private research organizations
conduct research and may market commercial products on their own or through
joint ventures. These institutions are becoming more active in seeking patent
protection and licensing arrangements to collect royalties for use of technology

                                      -14-

<PAGE>

that they have developed. These institutions also compete with us in recruiting
and retaining highly qualified scientific personnel.

     In particular, each of Avant Immunotherapeutics, Inc., Chiron Corporation,
Abbott Laboratories, Gliatech Inc. and Biocryst Pharmaceuticals has publicly
announced intentions to develop complement inhibitors to treat diseases related
to trauma, inflammation or neurodegenerative indications. We are aware that
SmithKline Beecham PLC, Merck & Co., Inc. and CytoMed Inc. are attempting to
develop similar therapies. In addition, each of Bayer A.G., Immunex Corporation,
Pharmacia & Upjohn and Rhone-Poulenc Rorer, Inc. sells a product which is used
to reduce surgical bleeding during CPB. We are also aware of announced and
ongoing clinical trials of certain companies, including Autoimmune, Inc., Immune
Response Corporation, Neurocrine Biosciences, Inc. and Anergen, Inc., employing
T-cell specific tolerance technologies and addressing patients with multiple
sclerosis or diabetes mellitus. Baxter Healthcare Corporation and Novartis,
Inc., in collaboration with Biotransplant Inc., have publicly announced
intentions to commercially develop xenograft organs. We are aware that Diacrin
Inc. and Genzyme Tissue Repair, Inc. are also working in this field. These
companies may succeed in developing products which are more effective or less
costly than our products. These companies may also be more successful in
producing and marketing their products. Competition may increase further as a
result of potential advances in the commercial applicability of biotechnology
and greater availability of capital for investment in these fields.

     DEPENDENCE ON QUALIFIED PERSONNEL. We are highly dependent upon the efforts
of our senior management and scientific personnel and, in particular, Dr.
Leonard Bell, our President and Chief Executive Officer. The loss of the
services of one or more of these individuals could materially and adversely
affect our ability to achieve our development objectives. Dr. Bell has an
employment agreement which expires on April 1, 2000. We have a $2,000,000 key
man life insurance policy on the life of Dr. Bell, naming us as the beneficiary.
Because of the specialized scientific nature of our business, we are also highly
dependent upon our ability to continue to attract and retain qualified
scientific and technical personnel. There is intense competition for qualified
scientific and technical personnel. We may not be able to continue to attract
and retain the qualified personnel necessary for developing our business. Loss
of the services of, or failure to recruit, key scientific and technical
personnel would be significantly detrimental to our product development
programs.

     All of our scientific consultants are employed on a full-time basis by
academic or research institutions or may have their own professional practices
or firms on a full-time basis. Accordingly, these consultants will be able to
devote only a limited portion of their time to us. In addition, in certain
circumstances, inventions or processes discovered by them may not become our
property but may be the property of their full-time employers or of other
companies and institutions for which they now consult. We may not be able to
negotiate license rights to the results of collaborations on commercially
reasonable terms, if at all.

     DEPENDENCE ON OUTSIDE PARTIES AND COLLABORATORS. For the research,
development, manufacture and commercialization of certain of our products, we
contemplate entering into various arrangements with corporate partners,
licensors, licensees, outside researchers, consultants and others. Therefore,
our success may depend in part upon the efforts of outside parties. We cannot
assure you that we will be able to negotiate acceptable collaborative
arrangements to develop or commercialize our products, that arrangements or
other collaborations entered into, if any, will be successful, or that current
or potential collaborators will not pursue treatments for other diseases or seek
alternative means of developing treatments for the diseases targeted by programs
with us. 

     For example, in January 1999, we entered into a collaboration agreement
with Proctor & Gamble Pharmaceuticals to develop and commercialize 5G1.1-SC.
However, we cannot assure you that we, together

                                      -15-

<PAGE>

with Proctor & Gamble, will successfully develop or commercialize 5G1.1-SC.
Moreover, we cannot assure you that we will receive all of the benefits
available to us under the collaboration agreement. Further, we cannot control
the amount and timing of funds or other resources that Proctor & Gamble will
devote to this collaboration.

     If any of our collaborators breaches or terminates its agreement with us or
otherwise fails to conduct its collaborative activities in a timely manner, the
development or commercialization of the product candidate or the research
program which is the subject of the agreement may be delayed. We may be required
to undertake unforeseen additional responsibilities or devote additional
resources to development or commer cialization or terminate the development or
commercialization. This could have a material adverse effect on our prospects,
financial condition, intellectual property position and results of operations.

     Although we began developing our Unigraft products in collaboration with
United States Surgical Corporation, we are currently in the process of
reacquiring their rights to all aspects of the xenotransplantation program from
Tyco International Ltd., which acquired United States Surgical Corporation. We
believe that, despite the termination of our collaboration with United States
Surgical Corporation, we will be able to continue to develop xenotransplantation
products, although we cannot be certain.

     LIMITED MANUFACTURING, MARKETING, SALES, CLINICAL TESTING AND REGULATORY
COMPLIANCE CAPABILITY. We have not invested in the development of commercial
manufacturing, marketing, distribution or sales capabilities. Moreover, we have
insufficient capacity to manufacture more than one product candidate at a time
or to manufacture its product candidates for later stage clinical development or
commercialization. If we are unable to develop or contract for additional
manufacturing capabilities on acceptable terms, our ability to conduct human
clinical testing will be materially adversely affected, resulting in delays in
the submission of products for regulatory approval and in the initiation of new
development programs. This could have a material adverse effect on our
competitive position and our prospects for achieving profitability. In addition,
as our product development efforts progress, we may need to hire additional
personnel skilled in clinical testing, regulatory compliance and, if we develop
products with commercial potential, marketing and sales. We cannot assure you
that we will be able to acquire, or establish third-party relationships to
provide, any or all of these resources.

     UNCERTAINTY OF AVAILABILITY OF HEALTHCARE REIMBURSEMENT. Our ability to
commercialize our products successfully may depend in part on the extent to
which reimbursement for the cost of such products and related treatments will be
available from government health administration authorities, private health
insurers and other organizations. Third-party payors are attempting to control
costs by limiting coverage of products and treatments and the level of
reimbursement for medical products and services. Significant uncertainty exists
as to the reimbursement status of newly approved healthcare products. If we
succeed in bringing one or more products to market, these products may not be
considered cost-effective, reimbursement may not be available, or, if available,
the payor's reimbursement policies may materially adversely affect our ability
to sell our products on a profitable basis.

     PRODUCT LIABILITY; POTENTIAL LIABILITY FOR HUMAN CLINICAL TRIALS. Our
business exposes us to potential product liability risks which are inherent in
the testing, manufacturing, marketing and sale of human therapeutic products. We
cannot assure you that we will be able to avoid significant product liability
exposure. With respect to our UniGraft program, little is known about the
potential long-term health risks of transplanting non- human tissue into humans.
In addition to product liability risks associated with sales of products, we may
be liable to the claims of individuals who participate in human clinical trials
of our products. While we have obtained and will seek waivers of liability from
all persons who participated or may 

                                      -16-

<PAGE>

in the future participate in human clinical trials conducted by or on our
behalf, we cannot assure you that waivers will be effective to protect us from
liability or the costs of product liability litigation. We currently have
product liability insurance to cover certain liabilities relating to the conduct
of human clinical trials. We may not be able to maintain our insurance on
acceptable terms, and our insurance may not provide adequate protection against
potential liabilities. Inadequate insurance may affect our ability to develop
and commercialize our products. Furthermore, a product liability lawsuit or
recall could have a material adverse effect on our business, financial condition
and results of operations.

     VOLATILITY OF SHARE PRICE. The market prices for securities of
biopharmaceutical companies have been volatile. Factors such as announcements of
technological innovations or new commercial products by us or our competitors,
government regulation, patent or proprietary rights developments, public concern
as to the safety or other implications of biopharmaceutical products, results of
preclinical or clinical trials, positive or negative developments related to our
collaborators and market conditions in general may have a significant impact on
the market price of our common stock.

     DILUTIVE EFFECT OF STOCK ISSUANCES, GRANTS, OPTIONS AND WARRANTS. As of
December 31, 1998, we have granted options to purchase an aggregate of
approximately 1,965,000 shares of our common stock under certain stock option
plans. Warrants to purchase an aggregate of 220,000 shares of our common stock
are also outstanding under previous financing arrangements and other
transactions. Many of these options have exercise prices below the current
market price of our common stock. In addition, we may issue additional stock,
warrants and/or options to raise capital in the future. We regularly examine
opportunities to expand our technology base through means such as licenses,
joint ventures and acquisition of assets or ongoing businesses and may issue
securities in connection with these transactions. We may also issue additional
securities in connection with our stock option plans. During the terms of these
options and warrants, the holders are given the opportunity to profit from a
rise in the market price of our common stock. The exercise of these options and
warrants may have an adverse effect on the market value of our common stock. The
existence of these options and warrants may adversely affect the terms on which
we can obtain additional equity financing. If the exercise prices of these
options and warrants are less than the net tangible book value of our common
stock at the time these options and warrants are exercised, our stockholders
will experience an immediate dilution in the net tangible book value of their
investment.

     POSSIBLE ADVERSE IMPACT ON HOLDERS OF COMMON STOCK; ANTI-TAKEOVER 
PROVISIONS; RIGHTS PLAN. The Board of Directors may issue one or more series 
of preferred stock, without any action on the part of our stockholders on 
terms which may adversely affect the rights of holders of common stock. 
Issuance of preferred stock may dilute the voting power of holders of common 
stock (such as by issuing preferred stock with super voting rights) and may 
render more difficult the removal of current management, even if removal may 
be in the stockholders' best interests. Further, the issuance of preferred 
stock may be used as an "anti-takeover" device without further action on the 
part of the stockholders. On February 14, 1997, the Board of Directors 
declared a dividend distribution of one preferred stock purchase right for 
each outstanding share of our common stock. These rights are not exercisable 
until the date of the earlier to occur of (i) ten business days following the 
time of a public announcement or notice to us that a person or group of 
affiliated or associated persons has acquired beneficial ownership of 20% or 
more of the outstanding shares of our common stock or (ii) ten business days, 
or a later date as may be determined by the Board of Directors, after the 
date of the commence ment or announcement by a person of an intention to make 
a tender offer or exchange offer for an amount of common stock which, 
together with the shares of common stock already owned by that person, 
constitutes 20% or more of the outstanding shares of common stock. The rights 
and the rights agreement, as well as certain provisions of Delaware law, are 
designed to prevent any unsolicited acquisitions of our common stock. These 
provisions and any issuance of preferred stock could prevent the holders of 
common stock from realizing a premium on their shares.

                                      -17-

<PAGE>

     OWNERSHIP BY MANAGEMENT AND PRINCIPAL STOCKHOLDERS. On February 1, 1999,
our directors and officers and certain principal stockholders and their
affiliates beneficially owned (as defined by the Commission) in the aggregate
approximately 1,994,918 shares of common stock, representing 16.8% of the
outstanding shares of common stock. Accordingly, they have the ability to
influence significantly our affairs and matters requiring a stockholder vote,
including the election of the directors, the amendment of charter documents, the
merger or dissolution of us and the sale of all or substantially all of our
assets. The voting power of these holders may also discourage or prevent any
proposed takeover of us pursuant to a tender offer.


                                 USE OF PROCEEDS

     We will not receive any proceeds from the sale of shares of common stock by
the selling stockholders, although we will receive the exercise prices of the
stock options.


                                      -18-


<PAGE>


                              SELLING STOCKHOLDERS

     The following table sets forth certain information with respect to certain
selling stockholders as of February 1, 1999. The shares are being registered to
permit public secondary trading of the shares, and the selling stockholders may
offer the shares for resale from time to time. See "Plan of Distribution."


<TABLE>
<CAPTION>

                                             Number of                                Number of Shares       Percent of Shares
                                              Shares               Number of             Beneficially           Beneficially
Name of Selling Stockholder(1)              Beneficially          Shares Being           Owned After            Owned After
- ------------------------------            Owned Prior to            OFFERED(2)             OFFERING              OFFERING
                                             OFFERING             -----------         ----------------       -----------------
                                          --------------
<S>                                       <C>                     <C>                 <C>                    <C>
John H. Fried, Ph.D. (3)                       90,636                11,500                 79,136                  *

Timothy F. Howe (4)                           288,154                 4,000                284,154                 2.5%

Max Link, Ph.D. (5)                            25,123                11,500                 13,623                  *

Joseph A. Madri, Ph.D., M.D. (6)               57,100                 4,000                 53,100                  *

Leonard Marks, Jr., Ph.D.  (7)                 15,600                11,500                  4,100                  *

Eileen M. More (8)                            521,650                 4,000                517,650                 4.6%
</TABLE>


- -------------------------

(1)  Unless otherwise indicated, the address of all persons is 25 Science Park,
     Suite 360, New Haven, Connecticut 06511.

(2)  To our knowledge, except as set forth below, the persons named in the table
     have sole voting and investment power with respect to all shares of common
     stock shown as beneficially owned by them, subject to community property
     laws where applicable and the information contained in the footnotes in
     this table.

(3)  Shares Beneficially Owned Prior to Offering include 14,966 shares of common
     stock that may be acquired on the exercise of options that are exercisable
     within 60 days of February 1, 1999, and exclude 3,334 shares of obtainable
     through the exercise of options which are not exercisable within 60 days of
     February 1, 1999.

(4)  Shares Beneficially Owned Prior to Offering include shares of common stock
     beneficially owned by Collinson Howe Venture Partners, Inc., and include
     5,766 shares which may be acquired upon the exercise of options within 60
     days of February 1, 1999, and exclude 5,034 shares obtainable through the
     exercise of options which are not exercisable within 60 days of February 1,
     1999. Mr. Howe disclaims beneficial ownership of shares held or
     beneficially owned by Collinson Howe Venture Partners, Inc.

     Collinson Howe Venture Partners, Inc. is a venture capital investment
     management firm, which is the managing member of Biotechnology Investment
     Group, L.L.C. As such, this investment firm shares beneficial ownership of
     279,400 shares of common stock owned by Biotechnology Investment Group,
     L.L.C. Mr. Howe, a director of Alexion, is the Vice President and a
     minority stockholder of this investment firm. As such, he shares investment
     and voting power over the shares beneficially owned by Collinson Howe
     Venture Partners, Inc.

(5)  Shares Beneficially Owned Prior to Offering include 2,366 shares of common
     stock that may be acquired on the exercise of options that are exercisable
     within 60 days of February 1, 1999, and exclude 3,334 shares of common
     stock obtainable through the exercise of options which are not exercisable
     within 60 days of February 1, 1999.

(6)  Shares Beneficially Owned Prior to Offering include 5,766 shares of common
     stock that may be acquired on the exercise of options that are exercisable
     within 60 days of February 1, 1999, and exclude 5,034 shares obtainable
     through the exercise of options which are not exercisable within 60 days of
     February 1, 1999.

                                      -19-

<PAGE>

(7)  Shares Beneficially Owned Prior to Offering include 14,966 shares of common
     stock which may be acquired upon the exercise of options within 60 days of
     February 1, 1999, and exclude 3,334 shares obtainable through the exercise
     of options which are not exercisable within 60 days of February 1, 1999.

(8)  Shares Beneficially Owned Prior to Offering include 25,766 shares of common
     stock which may be acquired upon the exercise of options within 60 days of
     February 1, 1999, and include 484,977 shares owned by Oak Investment V
     Partners and 10,907 shares owned by Oak Investment V Affiliates, two
     affiliated limited partnerships, exclude 5,034 shares obtainable through
     the exercise of options which are not exercisable within 60 days of
     February 1, 1999. Ms. More is a General Partner of these limited
     partnerships.

     We may supplement this prospectus from time to time to include certain
information concerning the security ownership of the selling stockholders and
the position, office or other material relationship which a selling stockholder
has had within the past three years with us or any of our affiliates; provided,
however, that certain unnamed non-affiliates, each of whom may sell up to the
lesser of 1,000 shares or one percent of the shares covered by this prospectus,
may use this prospectus for reoffers and resales.


                              PLAN OF DISTRIBUTION

     We are registering the shares covered by this prospectus on behalf of the
selling stockholders. All costs, expenses and fees in connection with the
registration of these shares will be paid by us. Brokerage commissions, if any,
attributable to the sale of these shares will be paid by the selling
stockholders or their donees or pledgees.

     Sales of these shares may be effected from time to time in transactions
(which may include block transactions) on the Nasdaq National Market, in
negotiated transactions, or a combination of such methods of sale, at fixed
prices which may be changed, at market prices prevailing at the time of sale, or
at negotiated or other prices. The selling stockholders may also sell these
shares pursuant to Rule 144 promulgated under the Securities Act of 1933, as
amended, or may pledge shares as collateral for margin accounts and such shares
could be resold pursuant to the terms of such accounts. Pursuant to this
prospectus, the selling stockholders may also donate a certain DE MINIMUS number
(as allowed by the Securities and Exchange Commission) of their shares of common
stock, and such shares could be resold pursuant to rules set forth by the
Commission. The selling stockholders have advised us that they have not entered
into any agreements, understandings or arrangements with any underwriters or
broker-dealers regarding the sale of their securities. The selling stockholders
may effect such transactions by selling common stock directly to purchasers or
to or through broker-dealers which may act as agents or principals. These
broker-dealers may receive compensation in the form of discounts, concessions or
commissions from each selling stockholder and/or the purchasers of the shares
for whom the broker-dealers may act as agents or to whom they sell as principal,
or both (which compensation as to a particular broker-dealer might be in excess
of customary commissions). The selling stockholders and any broker-dealers that
act in connection with the sale of the shares might be deemed to be
"underwriters" within the meaning of Section 2(11) of the Securities Act and any
commission received by them and any profit on the resale of the shares of common
stock as principal might be deemed to be underwriting discounts and commissions
under the Securities Act. The selling stockholders may agree to indemnify any
agent, dealer or broker-dealer that participates in transactions involving sales
of the shares against certain liabilities, including liabilities arising under
the Securities Act. Liabilities under the federal securities laws cannot be
waived.

     Because the selling stockholders may be deemed to be "underwriters" within
the meaning of Section 2(11) of the Securities Act, the selling stockholders
will be subject to prospectus delivery requirements under 

                                      -20-

<PAGE>

the Securities Act. Furthermore, in the event of a "distribution" of the shares,
the selling stockholder, any selling broker or dealer and any "affiliated
purchasers" may be subject to Regulation M under the Exchange Act. Such
regulation would prohibit, with certain exceptions, any such person from bidding
for or purchasing any security which is the subject of the distribution until
his, her or its participation in that distribution is completed. In addition,
Regulation M prohibits any "stabilizing bid" or "stabilizing purchase" for the
purpose of pegging, fixing or stabilizing the price of common stock in
connection with this offering.

                                  LEGAL MATTERS

     Legal matters relating to the common stock have been passed upon for us by
Fulbright & Jaworski L.L.P., New York, New York.

                                     EXPERTS

         Our audited financial statements incorporated by reference in this
prospectus and elsewhere in the registration statement have been audited by
Arthur Andersen LLP, independent public accountants, as indicated in their
report with respect thereto, and are included herein in reliance upon the
authority of said firm as experts in accounting and auditing in giving said
reports.

                                      -21-


<PAGE>

- -------------------------------------------------------------------------------
- -------------------------------------------------------------------------------

     NO PERSON (INCLUDING ANY SALESMAN OR BROKER) IS AUTHORIZED TO PROVIDE ORAL
OR WRITTEN INFORMATION ABOUT THIS OFFERING NOT CONTAINED IN THIS PROSPECTUS. YOU
SHOULD NOT ASSUME THAT THE INFORMATION CONTAINED IN THIS PROSPECTUS IS ACCURATE
AS OF ANY DATE OTHER THAN THE DATE INDICATED BELOW.

                    -------------
                  TABLE OF CONTENTS

<TABLE>
<CAPTION>
                                                                       
                                                       Page
                                                       ----
<S>                                                   <C>
Where You Can Find More Information .................... 2

Cautionary Statement Regarding
Forward-Looking Statements.............................. 3                  

Prospectus Summary...................................... 4

Risk Factors............................................10

Use of Proceeds.........................................18                    

Selling Stockholders....................................19                  

Plan of Distribution....................................20

Legal Matters ..........................................21
                                                                          
Experts.................................................21
</TABLE>


- -------------------------------------------------------------------------------
- -------------------------------------------------------------------------------

- -------------------------------------------------------------------------------
- -------------------------------------------------------------------------------



                                 200,000 Shares



                         ALEXION PHARMACEUTICALS, INC.



                                  COMMON STOCK




                                   PROSPECTUS




                               FEBRUARY 8, 1999

- -------------------------------------------------------------------------------
- -------------------------------------------------------------------------------

<PAGE>

                                     PART II

               INFORMATION REQUIRED IN THE REGISTRATION STATEMENT


ITEM 3.   INCORPORATION OF DOCUMENTS BY REFERENCE.

     The information in the following documents which we have filed with the
Commission (File No. 0- 27756) pursuant to the Exchange Act is incorporated by
reference in this Registration Statement:

     (i)   Our Annual Report on Form 10-K for the fiscal year ended July 31,
           1998;

     (ii)  Our Quarterly Report on Form 10-Q for the quarter ended October 31,
           1998;

     (iii) Our Current Reports on Form 8-K, filed on October 9, 1998, December
           31, 1998 and January 29, 1999; and

     (iv)  Our Registration Statement on Form 8-A, dated February 12, 1996.

All documents and reports subsequently filed by us pursuant to Sections 13(a),
13(c), 14 or 15(d) of the Exchange Act after the date of this Registration
Statement and prior to the termination of this offering is incorporated by
reference into this Registration Statement and will be a part of this
Registration Statement from the date of the filing of those documents or
reports. The information relating to us in this Registration Statement should be
read together with the information in the documents incorporated by reference.

     Any statement contained in a document incorporated by reference herein,
unless otherwise indicated therein, speaks as of the date of the document.
Statements contained in this Registration Statement may modify or replace
statements contained in the documents incorporated by reference.

ITEM 4.   DESCRIPTION OF SECURITIES.

     Not applicable.

ITEM 5.   INTEREST OF NAMED EXPERTS AND COUNSEL.

     Not applicable.

ITEM 6.   INDEMNIFICATION OF DIRECTORS AND OFFICERS.

     Section 145 of the General Corporation Law of the State of Delaware permits
indemnification of directors, officers and employees of a corporation under
certain conditions and subject to certain limitations. Our Certificate of
Incorporation provides that we shall, to the fullest extent permitted by Section
145, indemnify any and all persons whom it shall have power to indemnify under
said Section. In addition, we have entered into indemnity agreements with our
directors and officers providing for the maximum indemnification allowed by
Section 145.

ITEM 7.   EXEMPTION FROM REGISTRATION CLAIMED.

                                      II-1


<PAGE>

     Not applicable.

ITEM 8.   EXHIBITS.


<TABLE>
<CAPTION>

     Exhibit
        No.       Description
     -------      -----------
<S>               <C>
     4.1          Alexion Pharmaceuticals, Inc. 1992 Stock Option Plan For 
                  Outside Directors

     4.2          Form of Option Agreement

     5            Opinion of Fulbright & Jaworski L.L.P.

     23.1         Consent of Arthur Andersen LLP

     23.2         Consent of Fulbright & Jaworski L.L.P. (included in Exhibit 5)

     24           Power of Attorney (included on signature page)
</TABLE>


ITEM 9.   UNDERTAKINGS

     (a)  The undersigned registrant hereby undertakes:

          (1)  To file, during any period in which offers or sales are being
     made, a post-effective amendment to this registration statement:

               (i)  To include any prospectus required by Section 10(a)(3) of
     the Securities Act of 1933;

               (ii) To reflect in the prospectus any facts or events arising
     after the effective date of the registration statement (or the most recent
     post-effective amendment thereof) which, individually or in the aggregate,
     represent a fundamental change in the information set forth in the
     registration statement. Notwithstanding the foregoing, any increase or
     decrease in volume of securities offered (if the total dollar value of
     securities offered would not exceed that which was registered) and any
     deviation from the low or high end of the estimated maximum offering range
     may be reflected in the form of prospectus filed with the Commission
     pursuant to Rule 424(b) if, in the aggregate, the changes in volume and
     price represent no more than 20 percent change in the maximum aggregate
     offering price set forth in the "Calculation of Registration Fee" table in
     the effective registration statement;

               (iii) To include any material information with respect to the
     plan of distribution not previously disclosed in the registration statement
     or any material change to such information in the registration statement;
     PROVIDED, HOWEVER, that paragraphs (a)(1)(i) and (a)(1)(ii) do not apply if
     the information required to be included in a post-effective amendment by
     those paragraphs is contained in periodic reports filed by the registrant
     pursuant to Section 13 or Section 15(d) of the Securities Exchange Act of
     1934 that are incorporated by reference in the registration statement.

          (2) That, for the purpose of determining any liability under the
     Securities Act of 1933, each such post-effective amendment shall be deemed
     to be a new registration statement

                                      II-2

<PAGE>

     relating to the securities offered therein, and the offering of such
     securities at that time shall be deemed to be the initial BONA FIDE
     offering thereof.

          (3) To remove from registration by means of a post-effective amendment
     any of the securities being registered which remain unsold at the
     termination of the offering.

     (b) The undersigned registrant hereby undertakes that, for purposes of
determining any liability under the Securities Act of 1933, each filing of the
registrant's annual report pursuant to Section 13(a) or Section 15(d) of the
Securities Exchange Act of 1934 (and, where applicable, each filing of an
employee benefit plan's annual report pursuant to Section 15(d) of the
Securities Exchange Act of 1934) that is incorporated by reference in the
registration statement shall be deemed to be a new registration statement
relating to the securities offered therein, and the offering of such securities
at that time shall be deemed to be the initial BONA FIDE offering thereof.

     (c) The undersigned registrant hereby undertakes to deliver or cause to be
delivered with the prospectus, to each person to whom the prospectus is sent or
given, the latest annual report to security holders that is incorporated by
reference in the prospectus and furnished pursuant to and meeting the
requirements of Rule 14a-3 or Rule 14c-3 under the Securities Exchange Act of
1934; and, where interim financial information required to be presented by
Article 3 of Regulation S-X is not set forth in the prospectus, to deliver, or
cause to be delivered to each person to whom the prospectus is sent or given,
the latest quarterly report that is specifically incorporated by reference in
the prospectus to provide such interim financial information.

     (d) Insofar as indemnification for liabilities arising under the Securities
Act of 1933 may be permitted to directors, officers and controlling persons of
the registrant pursuant to the foregoing provisions, or otherwise, the 
registrant has been advised that in the opinion of the Securities and Exchange 
Commission such indemnification is against public policy as expressed in the 
Act and is, therefore, unenforceable. In the event that a claim for 
indemnification against such liabilities (other than the payment by the
registrant of expenses incurred or paid by a director, officer or controlling
person of the registrant in the successful defense of any action, suit or
proceeding) is asserted by such director, officer or controlling person in
connection with the securities being registered, the registrant will, unless in
the opinion of its counsel the matter has been settled by controlling precedent,
submit to a court of appropriate jurisdiction the question whether such
indemnification by it is against public policy as expressed in the Act and will
be governed by the final adjudication of such issue.

                                      II-3


<PAGE>

     (e) The undersigned registrant hereby undertakes that:

          (1) For purposes of determining any liability under the Securities Act
of 1933, the information omitted from the form of prospectus filed as part of
this registration statement in reliance upon Rule 430A and contained in a form
of prospectus filed by the registrant pursuant to Rule 424(b)(1) or (4) or
497(h) under the Securities Act shall be deemed to be part of this registration
statement as of the time it was declared effective.

          (2) For the purposes of determining any liability under the Securities
Act of 1933, each post-effective amendment that contains a form of prospectus
shall be deemed to be a new registration statement relating to the securities
offered therein, and the offering of such securities at that time shall be
deemed to be the initial BONA FIDE offering thereof.



                                      II-4


<PAGE>

                                   SIGNATURES

     Pursuant to the requirements of the Securities Act of 1933, as amended, the
registrant certifies that it has reasonable grounds to believe that it meets all
of the requirements for filing on Form S-8 and has duly caused this Registration
Statement to be signed on its behalf by the undersigned, thereunto duly
authorized, in the City of New Haven, State of Connecticut on February 8, 1999.


                                    ALEXION PHARMACEUTICALS, INC.

                                    By: /s/ Leonard Bell                  
                                        ------------------------
                                        Leonard Bell, M.D.
                                        President, Chief Executive Officer,
                                        Secretary and Treasurer


                               -----------------


                                POWER OF ATTORNEY

     KNOW ALL MEN BY THESE PRESENTS, that each individual whose signature
appears below constitutes and appoints LEONARD BELL, M.D. and DAVID W. KEISER,
or either of them, his true and lawful attorney-in-fact and agent with full
power of substitution and resubstitution, for him and in his name, place and
stead, in any and all capacities, to sign any and all amendments (including
post-effective amendments) to this Registration Statement and to file the same
with all exhibits thereto and all documents in connection therewith, with the
Securities and Exchange Commission, granting said attorney-in-fact and agent and
each of them, full power and authority to do and perform each and every act and
thing requisite and necessary to be done in and about the premises, as fully to
all intents and purposes as he or she might or could do in person, hereby
ratifying and confirming all that said attorney-in-fact and agent or any of
them, or their or his substitute or substitutes, may lawfully do or cause to be
done by virtue hereof.


                                      II-5


<PAGE>

     Pursuant to the requirements of the Securities Act of 1933, as amended,
this Registration Statement has been signed below by the following persons in
the capacities and on the dates indicated.


<TABLE>
<CAPTION>

<S>                             <C>                                      <C>
/s/ LEONARD BELL               President, Chief Executive Officer,       February 8, 1999
- ----------------------------   Secretary, Treasurer and Director
Leonard Bell, M.D.             (principal executive officer)

/s/ DAVID W. KEISER            Executive Vice President and              February 8, 1999
- ----------------------------   Chief Operating Officer
David W. Keiser                (principal financial officer)
                              
/s/ BARRY P. LUKE              Vice President of Finance                 February 8, 1999
- ----------------------------   and Administration
Barry P. Luke                  (principal accounting officer)           
                               
/s/ JOHN H. FRIED              Chairman of the Board of                  February 8, 1999
- ----------------------------   Directors
John H. Fried, Ph.D.      

/s/ TIMOTHY F. HOWE            Director                                  February 8, 1999
- ----------------------------
Timothy F. Howe

/s/ MAX LINK                   Director                                  February 8, 1999
- ----------------------------
Max Link, Ph.D.

/s/ JOSEPH A. MADRI            Director                                  February 8, 1999
- ----------------------------
Joseph A. Madri, Ph.D., M.D.

/s/ LEONARD MARKS              Director                                  February 8, 1999
- ----------------------------
Leonard Marks, Jr., Ph.D.

/S/ EILEEN M. MORE             Director                                  February 8, 1999
- ----------------------------
Eileen M. More
</TABLE>


                                      II-6


<PAGE>


                                INDEX TO EXHIBITS


<TABLE>
<CAPTION>

Exhibit
  No.          Description
- -------        -----------
<S>            <C>
4.1            Alexion Pharmaceuticals, Inc. 1992 Stock Option Plan For Outside
               Directors

4.2            Form of Option Agreement

5              Opinion of Fulbright & Jaworski L.L.P.

23.1           Consent of Arthur Andersen LLP

23.2           Consent of Fulbright & Jaworski L.L.P. (included in Exhibit 5)

24             Power of Attorney (included on signature page)

</TABLE>






<PAGE>


                                                                 Exhibit 4.1

                          ALEXION PHARMACEUTICALS, INC.
                  1992 STOCK OPTION PLAN FOR OUTSIDE DIRECTORS


I        PURPOSE.

         The purpose of this 1992 Stock Option Plan for Outside Directors (the
"Plan") of Alexion Pharmaceuticals, Inc. (the "Corporation") is to enable the
Corporation to compensate eligible directors of the Corporation and to encourage
the highest level of performance by providing such persons with a proprietary
interest in the Corporation's success and progress by granting them shares of
the Corporation's Common Stock, par value $.0001 per share ("Common Stock").

II       ADMINISTRATION OF THE PLAN

         The Plan shall be administered by a committee (the "Committee") of the
Board of Directors of the Corporation (the "Board"), which shall consist of one
or more members of the Board, appointed by the Board, who are outside directors
(as defined below) or by the Board. The interpretation and construction by the
Committee of any provisions of the Plan or of any other matters related to the
Plan shall be final. The Committee may from time to time adopt such rules and
regulations for carrying out the Plan as it may deem advisable. No member of the
Board or the Committee shall be liable for any action or determination made in
good faith with respect
 to the Plan. The Plan shall be interpreted and
implemented such that the eligible outside directors will not fail, by reason of
the Plan or its implementation, to be "disinterested persons" within the meaning
of Rule 16b-3 of the Securities Exchange Act of 1934 (the "Exchange Act"), as
such Rule and such Act may be amended.

III      ELIGIBILITY AND ISSUANCES.

         A.   ELIGIBILITY.   Directors of the Corporation who (i) are neither
officers nor employees nor consultants of the Corporation or any of its
subsidiaries (other than the Chairman of the Board of Directors of the
Corporation who shall be eligible) and (ii) are not affiliated with any person
referred to in (i) above ("outside directors") shall be eligible to receive
options to purchase Common Stock under the Plan.

         B.   ISSUANCES

              (1)  Except as set forth in Section 3(b)(ii) below, each outside
         director shall be issued an option to purchase 7,500 shares of the
         Corporation's Common Stock (the "Initial Option") on the date of his
         initial election or appointment to the Board of Directors (the "Initial
         Grant Date") on the following terms:

                   (a)  The option exercise price per share of Common Stock
              shall be the Fair Market Value (as defined below) of the Common
              Stock covered by such Initial Option on the Initial Grant Date.

                   (b)  Except as provided herein, the term of an Initial Option
              shall be for a period of ten (10) years from the Initial Grant
              Date.


                                       -1-


<PAGE>


              (2)  In the case of Drs. John A. Fried, Max Link and Leonard
         Marks, Jr., outside directors who were elected as directors of the
         Corporation in April 1992, Initial Options to purchase 7,500 (after
         giving effect to a 1 for 4 reverse stock split effected on November 7,
         1994 and a 1 for 2.5 reverse stock split effected on January 5, 1996)
         shares of the Corporation's Common Stock were granted under the Plan on
         August 27, 1992, the date of the Plan's adoption by the Board.

              (3)  In addition, each outside director shall, on the date of each
         annual meeting of stockholders at which he is reelected as a director
         (the "Additional Grant Date"), if he is still an outside director on
         such date and has attended, either in person or by telephone, at least
         seventy-five percent (75%) of the meetings of the Board of Directors
         that were held while he was a director since the prior annual meeting
         of stockholders, be granted an option to purchase 2,000 shares of
         Common Stock (the "Additional Option" and, together with the Initial
         Option, an "Option") on the following terms:

                   (a)  The option exercise price per share of Common Stock
              shall be the Fair Market Value (as defined below) of the Common
              Stock covered by such Additional Option on the Additional Grant
              Date.

                   (b)  Except as provided herein, the term of an Additional
              Option shall be for a period of ten (10) years from the Additional
              Grant Date.

              (4)  "Fair Market Value" shall mean, for each Initial Grant Date
         or Additional Grant Date (collectively, a "Grant Date"), (A) if the
         Common Stock is listed or admitted to trading on the New York Stock
         Exchange (the "NYSE") or the American Stock Exchange (the "ASE"), the
         last reported sale price of the Common Stock on such date or, if no
         sale takes place on such date, the closing asked prices of the Common
         Stock on such exchange as of such date, in each case as officially
         reported on the NYSE or the ASE, or (B) if no shares of Common Stock
         are then listed or admitted to trading on the NYSE or the ASE, the last
         reported sales price of the Common Stock on such date on the NASDAQ
         National Market System ("NASDAQ") or, if no shares of Common Stock are
         then quoted on NASDAQ, the average of the closing bid and the highest
         asked prices of the Common Stock on such date on NASDAQ, or, if no
         shares of Common Stock are then quoted on NASDAQ, the average of the
         highest bid and the lowest asked prices of the Common Stock on such
         date as reported on the over-the-counter system. If no closing bid and
         lowest asked prices thereof are then so quoted or published in the
         over-the-counter market, "Fair Market Value" shall mean the fair value
         per share of Common Stock (assuming for the purposes of this
         calculation the economic equivalence of all shares of classes of
         capital stock), as determined on a fully diluted basis in good faith by
         the Board, as of a date which is 15 days preceding the Grant Date;
         PROVIDED, HOWEVER, that the Fair Market Value of the Common Stock for
         purposes of Options granted prior to the closing of the Corporation's
         initial private placement of Common Stock or initial public offering,
         whichever occurs earliest, shall be the price per share of the Common
         Stock in such private placement or public offering.

              (5)  Options granted hereunder shall not be "incentive stock
         options" within the meaning of Section 422A of the Internal Revenue
         Code of 1986, as amended.


IV       REGULATORY COMPLIANCE AND LISTING.

                                      - 2 -


<PAGE>


         The issuance or delivery of any Option may be postponed by the
Corporation, and an Option shall not be exercisable, for such period as may be
required to comply with the Federal securities laws, state "blue sky" laws, any
applicable listing requirements of any applicable securities exchange and any
other law or regulation applicable to the issuance, delivery or exercise of such
Options and the Corporation shall not be obligated to issue or deliver any
Options or shares of Common Stock if the issuance or delivery of such Options or
shares would constitute a violation of any law or any regulation of any
governmental authority or applicable securities exchange.

V        RESTRICTIONS ON EXERCISABILITY.

         A.   Except as provided in Section 5(b) below, each Option granted
under the Plan may be exercisable as to one-third of the total number of shares
issuable under such Option on each of the three successive anniversaries of the
Grant Date of such Option.

         B.   If any event constituting a "Change in Control of the Corporation"
shall occur, all Options granted under the Plan, which are outstanding at the
time a Change of Control of the Corporation shall occur, shall immediately
become exercisable. A "Change in Control of the Corporation" shall be deemed to
occur if (i) there shall be consummated (x) any consolidation or merger of the
Corporation in which the Corporation is not the continuing or surviving
corporation or pursuant to which shares of the Corporation's Common Stock would
be converted into cash, securities or other property, other than a merger of the
Corporation in which the holders of the Corporation's Common Stock immediately
prior to the merger have the same proportionate ownership of common stock of the
surviving corporation immediately after the merger, or (y) any sale, lease,
exchange or other transfer (in one transaction or a series of related
transactions) of all, or substantially all, of the assets of the Corporation, or
(ii) the stockholders of the Corporation shall approve any plan or proposal for
liquidation or dissolution of the Corporation, or (iii) any person (as such term
is used in Section 13(d) and 14(d)(2) of the Securities Exchange Act of 1934, as
amended (the "Exchange Act")), shall become the beneficial owner (within the
meaning of Rule 13d-3 under the Exchange Act) of 40% or more of the
Corporation's outstanding Common Stock other than pursuant to a plan or
arrangement entered into by such person and the Corporation, or (iv) during any
period of two consecutive years, individuals who at the beginning of such period
constitute the entire Board shall cease for any reason to constitute a majority
thereof unless the election, or the nomination for election by the Corporation's
stockholders, of each new director was approved by a vote of at least two-thirds
of the directors then still in office who were directors at the beginning of the
period.

VI       CESSATION AS DIRECTOR.

         In the event that the holder of an Option granted pursuant to the Plan
shall cease to be a director of the Corporation for any reason, such holder may
exercise any portion of such Option that is exercisable by him at the time he
ceases to be a director of the Corporation, but only to the extent such Option
is exercisable as of such date, within six months after the date he ceases to be
a director of the Corporation.

VII      DEATH.

         In the event that a holder of an Option granted pursuant to the Plan
shall die, his beneficiary may exercise any portion of such Option that was
exercisable by the deceased Optionee at the time of his death, but only to the
extent such Option is exercisable as of such date, within twelve months after
the date of his death.


                                      - 3 -


<PAGE>


VIII     STOCK SPLITS, MERGERS, ETC.

         In the event of any stock split, stock dividend or similar transaction
which increases or decreases the number of outstanding shares of Common Stock,
appropriate adjustment shall be made by the Board, whose determination shall be
final, to the number and option exercise price per share of Common Stock which
may be purchased under any outstanding Options. In the case of a merger,
consolidation or similar transaction which results in a replacement of the
Corporation's Common Stock and stock of another corporation but does not
constitute a Change in Control of the Corporation, the Corporation will make a
reasonable effort, but shall not be required, to replace any outstanding Options
granted under the Plan with comparable options to purchase the stock of such
other corporation, or will provide for immediate maturity of all outstanding
Options, with all Options not being exercised within the time period specified
by the Board of Directors being terminated.

IX       TRANSFERABILITY.

         Options are not assignable or transferable, except by will or the laws
of descent and distribution to the extent set forth in Section 7 and during a
director's lifetime may be exercised only by him.

X        EXERCISE OF OPTIONS.

         An optionholder electing to exercise an Option shall give written
notice to the Corporation of such election and of the number of shares of Common
Stock that he has elected to acquire. An optionholder shall have no rights of a
stockholder with respect to shares of Common Stock covered by his Option until
after the date of issuance of a stock certificate to him upon partial or
complete exercise of his option.

XI       PAYMENT.

         The Option exercise price shall be payable in cash, check or in shares
of Common Stock upon the exercise of the Option. If the shares of Common Stock
are tendered as payment of the Option exercise price, the value of such shares
shall be the Fair Market Value as of the date of exercise. If such tender would
result in the issuance of fractional shares of Common Stock, the Corporation
shall instead return the difference in cash or by check to the employee.

XII      TERM OF PLAN.

         The Plan shall terminate on August 26, 2002, and no Option shall be
granted pursuant to the Plan after that date.

XIII     OBLIGATION TO EXERCISE OPTION.

         The granting of an Option shall impose no obligation on the director to
exercise such Option.

XIV      CONTINUANCE AS DIRECTOR.

         Nothing in the Plan shall be deemed to create any obligation on the
part of the Board to nominate any director for reelection by the Corporation's
stockholders.


                                      - 4 -


<PAGE>


XV       AMENDMENT OF THE PLAN.

         The Board may at any time and from time to time alter, amend, suspend
or terminate the Plan in whole or in part, provided, however, that (i) any
amendment which must be approved by the stockholders of the Company in order to
maintain the continued qualification of the Plan under Rule 16b-3 under the
Exchange Act or any successor provision, or the approval of which is otherwise
required by law, shall not be effective unless and until such stockholder
approval has been obtained in compliance with such rule or law and (ii)
provisions of the Plan which govern the amount, price or timing of the award of
an Option shall not be amended more than once every six months, other than to
comply with changes in the Internal Revenue Code of 1986, as amended, the
Employee Income Retirement Security Act, or the rules thereunder.

XVI      WITHHOLDING OF TAXES.

         We shall have the right, prior to the delivery of any certificate
evidencing shares of Common Stock to be issued pursuant to an Option, to require
the exercising outside director to remit to the Company an amount in cash
sufficient to satisfy any Federal, state, or local tax withholding requirements.





<PAGE>


                                                                  Exhibit 4.2

                        ALEXION PHARMACEUTICALS, INC.
                           STOCK OPTION AGREEMENT
                           ----------------------


        AGREEMENT made as of the ____ day of _________ 1998 by and between 
Alexion Pharmaceuticals, Inc., a Delaware corporation (the "Company") and   
(the "Optionee").

                            W I T N E S S E T H
                            -------------------

        WHEREAS, pursuant to the Alexion Pharmaceuticals, Inc. 1992 Stock 
Option Plan for Outside Directors (the "Plan"), the Company desires to grant 
to the Optionee and the Optionee desires to accept an option to purchase 
shares of common stock, $.0001 par value, of the Company (the "Common Stock") 
upon the terms and conditions set forth in this agreement;

        NOW, THEREFORE, the parties hereto agree as follows:

        1. GRANT.  The Company hereby grants to the Optionee an option to 
purchase _________ shares of Common Stock, at a purchase price per share of 
$________.  This option is intended to be treated as an option which does not 
qualify as an incentive stock option ("NSO") within the meaning of Section 
422 of the Internal Revenue Code of 1986.

        2. RESTRICTIONS ON EXERCISABILITY. Except as specifically provided 
otherwise herein, the option will become exercisable in accordance with the 
following schedule based upon the number of full years of the Optionee's 
continuous employment or service with the Company or a subsidiary following 
the Grant
 Date:



<PAGE>

    Full                 Incremental     Cumulative
    Years of Continuous  Percentage of   Percentage of
    Employment/          Option          Option
    Service              Exercisable     Exercisable
    -------------------  -------------   -------------
       Less than 1          0%              0%
                 1          33 1/3%         33 1/3%
                 2          33 1/3%         66 2/3%
                 3          33 1/3%         100%

No share of Common Stock may be purchased hereunder unless the Optionee shall 
have remained in the continuous employ or service of the Company or a 
subsidiary for one year from the Grant Date.  If the Optionee performs 
services for the Company or a subsidiary in a capacity other than as a 
director or employee, then, for purposes hereof, those services will be 
deemed to be continuous until they are terminated, and they will be deemed to 
be terminated at the time provided therefor in the consulting or other 
agreement governing the performance of such services or, if there is no such 
agreement, at the time the Company notifies the Optionee that it no longer 
contemplates the utilization of such services.  Unless sooner terminated, the 
option will expire if and to the extent it is not exercised within ten years 
from the Grant Date.

        3. EXERCISE. The option may be exercised in whole or in part in 
accordance with the above schedule by delivering to the Secretary of the 
Company (a) a written notice specifying the number of shares to be purchased, 
and (b) payment in full of the exercise price, together with the amount, if 
any, deemed necessary by the Company to enable it to satisfy any income tax 
withholding obligations with respect to the exercise (unless other 
arrangements, acceptable to the Company, are made for the satisfaction of 
such withholding obligations).  The exercise price shall be payable by bank 
or certified check.  The Company may (in its sole and absolute discretion) 
permit all or part of the exercise price to be paid with previously-owned 
shares of Common Stock, or in installments (together with interest) evidenced 
by the Optionee's secured promissory note.

        4. RIGHTS AS STOCKHOLDER.  No shares of Common Stock shall be sold or 
delivered hereunder until full payment for such shares has been made


<PAGE>

(or, to the extent payable in installments, provided for).  The Optionee 
shall have no rights as a stockholder with respect to any shares covered by 
the option until a stock certificate for such shares is issued to him or her. 
Except as otherwise provided herein, no adjustment shall be made for 
dividends or distributions of other rights for which the record date is prior 
to the date such stock certificate is issued.

        5. NONTRANSFERABILITY.  The option is not assignable or transferable 
except upon the Optionee's death to a beneficiary designated by the Optionee 
or, if no designated beneficiary shall survive the Optionee, pursuant to the 
Optionee's will and/or the laws of descent and distribution.  During an 
Optionee's lifetime, the option may be exercised only by the Optionee or the 
Optionee's guardian or legal representative.

        6. TERMINATION OF SERVICE, DISABILITY OR DEATH.  If the Optionee 
ceases to be employed by or to perform services for the Company and any 
subsidiary for any reason other than death or disability, then, unless sooner 
terminated under the terms hereof, the option will terminate on the date 
three months after the date of the Optionee's termination of employment or 
service.  If the Optionee's employment or service is terminated by reason of 
the Optionee's death or disability (or if the Optionee's employment or 
service is terminated by reason of his or her disability and the Optionee 
dies within one year after such termination of employment or service), then, 
unless sooner terminated under the terns hereof, the option will terminate on 
the date one year after the date of such termination of employment or service 
(or one year after the Optionee's later death).

        7. SECURITIES RESTRICTIONS.  If the shares to be issued upon an 
exercise of the option are not registered under the Securities Act of 1933, 
then, as a further condition of the Company's obligation to issue such 
shares, the Optionee may be required to give a representation in writing that 
the Optionee is acquiring the shares for his or her own account as an 
investment and not with a view to, or for sale in connection with, the 
distribution of such shares, and the certificates representing such shares 
shall bear a legend to such effect as the company's counsel shall deem 
necessary or desirable.  The option shall in no event be exercisable and 
shares shall not be issued hereunder if, in the opinion


<PAGE>

of counsel to the Company, such exercise and/or issuance would result in a 
violation of federal or state securities laws.

        8.  CAPITAL CHANGES, REORGANIZATIONS, ETC.

            (a) In case of any post-Grant Date split-up or consolidation of 
shares or any like capital adjustment, or the payment of a stock dividend 
which increases or decreases the number of outstanding shares of Common 
Stock, appropriate adjustment shall be made to the number of shares and the 
exercise price per share which may still be purchased under this agreement.

            (b) Upon a post-Grant Date merger (other than a merger of the 
Company in which the holders of Common Stock immediately prior to the merger 
have the same proportionate ownership of common stock in the surviving 
corporation immediately after the merger), consolidation, sale of property or 
stock, separation, reorganization (other than a mere reincorporation or the 
creation of a holding company) or liquidation of the Company, as a result of 
which the stockholders of the Company receive cash, stock or other property 
in exchange for or in connection with their shares of Common Stock an 
("Exchange Transaction"), the Optionee will be permitted to exercise his or 
her outstanding option (whether or not otherwise exercisable) and any 
outstanding options not exercised before the consummation of the Exchange 
Transaction will thereupon terminate.  Notwithstanding the preceding 
sentence, if, as part of the Exchange Transaction, the shareholders of the 
Company receive capital stock of another corporation ("Exchange Stock"), and 
if the Board, in its sole discretion, so directs, then all outstanding 
options will be converted into options to purchase shares of Exchange Stock.  
The amount and price of the converted options will be determined by adjusting 
the amount and price of the options granted hereunder on the same basis as 
the determination of the number of shares of Exchange Stock the holders of 
Common Stock will receive in the Exchange Transaction.

           (c) In the event of any adjustment in the number of shares covered 
by any option pursuant to the provisions hereof, any fractional shares 
resulting from such adjustment will be disregarded and each such option will 
cover only the number of full shares resulting from the adjustment.


<PAGE>

           (d) All adjustments under this paragraph 8 shall be made by the 
Board, and its determination as to what adjustments shall be made, and the 
extent thereof, shall be final, binding and conclusive.

        9.  NO EMPLOYMENT RIGHTS.  Nothing in this agreement shall give the 
Optionee any right to continue in the employ or service of the Company or a 
subsidiary, or interfere in any way with the right of the Company to 
terminate the employment or service of the Optionee.

        10. PROVISIONS OF PLAN. The provisions of the Plan shall govern if an 
to the extent that there are inconsistencies between those provisions and the 
provisions hereof.  The Optionee acknowledges that he or she has received a 
copy of the Plan prior to the execution of this agreement.

        11. ADMINISTRATION. The committee appointed by the Board to 
administer the Plan will have full power and authority to interpret and apply 
the provisions of this agreement, and the decision of said committee as to 
any matter arising under this agreement  shall be binding and conclusive as 
to all persons.

        12. TERMINATION OF GRANT. Prior to the Grant Date, the Board of 
Directors if, and to the extent necessary, in order to successfully 
consummate the Company's private placement, may not grant the option, and to 
that extent, the Optionee will have no further rights hereunder.

        13. MISCELLANEOUS.

            (a) This agreement shall be binding upon and shall inure to the 
benefit of the parties hereto and their respective successors and permitted 
assigns.

            (b) This agreement shall be governed by and construed in 
accordance with the laws of the State of Delaware.  This agreement 
constitutes the entire agreement between the parties with respect to the 
Subject matter hereof and may not be modified except by written instrument 
executed by the parties.


<PAGE>

        IN WITNESS WHEREOF, this agreement has been executed as of the date 
first above written

                                  Alexion Pharmaceuticals, Inc.


                                  By:
                                     -----------------------------------



                                  --------------------------------------
                                  Optionee





<PAGE>

                  [Fulbright & Jaworski L.L.P. Letterhead]


                                                                   Exhibit 5


                                                            February 8, 1999


Alexion Pharmaceuticals, Inc.
25 Science Park
New Haven, Connecticut  06511

Dear Sirs or Madams:

         We refer to the Registration Statement on Form S-8 (the "Registration
Statement") to be filed with the Securities and Exchange Commission under the
Securities Act of 1933, as amended (the "Act"), on behalf of Alexion
Pharmaceuticals, Inc. (the "Company"), relating to 200,000 shares of the
Company's Common Stock, $.0001 par value per share (the "Shares"), to be issued
under the Alexion Pharmaceuticals, Inc. 1992 Stock Option Plan for Outside
Directors, as amended (the "Plan").

         As counsel for the Company, we have examined such corporate records,
other documents, and such questions of law as we have considered necessary or
appropriate for the purposes of this opinion and, upon the basis of such
examination, advise you that, in our opinion, all necessary corporate
proceedings by the Company have been duly taken to authorize the issuance of the
Shares pursuant to the Plan and that the Shares being registered pursuant to the
Registration Statement, when issued and paid for under the Plan in accordance
with the terms of the Plan, will be duly authorized, validly issued, fully paid

and non-assessable.

         We hereby consent to the filing of this opinion as an exhibit to the
Registration Statement. This consent is not be construed as an admission that we
are a person whose consent is required to be filed with the Registration
Statement under the provisions of the Act.



                                  Very truly yours,


                                  /s/ Fulbright & Jaworski L.L.P.



<PAGE>

                                                                Exhibit 23.1


                    CONSENT OF INDEPENDENT PUBLIC ACCOUNTANTS


As independent public accountants, we hereby consent to the incorporation by
reference in this registration statement of our report dated August 28, 1998
included in Alexion Pharmaceuticals, Inc.'s Form 10-K for the year ended July
31, 1998 and to all references to our Firm included in this registration
statement.

                                  /S/ ARTHUR ANDERSEN LLP

                                  ARTHUR ANDERSEN LLP


Hartford, Connecticut
February 3, 1999