<PAGE>

    As filed with the Securities and Exchange Commission on February 5, 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
                         (State or Other Jurisdiction of
                         Incorporation or Organization)

                                   13-3648318
                                (I.R.S. Employer
                               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
                            (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                     
                                                                                MAXIMUM              MAXIMUM                      
                                                                             OFFERING PRICE         AGGREGATE        AMOUNT OF
TITLE OF SECURITIES                                    AMOUNT TO BE               PER                OFFERING       REGISTRATION
  TO BE REGISTERED                                      REGISTERED              SHARE(1)             PRICE(1)           FEE
- ----------------------------------------------------------------------------------------------------------------------------------
- ----------------------------------------------------------------------------------------------------------------------------------
<S>                                                <C>                       <C>                <C>               <C>      
Common Stock, par value of $.0001 per share...        1,300,000 shares          $13.375            $17,387,500       $4,835.00
- ----------------------------------------------------------------------------------------------------------------------------------
- ----------------------------------------------------------------------------------------------------------------------------------
</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 multiplying
         1,300,000, the number of additional shares registered by this
         Registration Statement as to which options may be granted under the
         Alexion Pharmaceuticals, Inc. 1992 Stock Option Plan, 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>





               INFORMATION REQUIRED IN THE REGISTRATION STATEMENT


         The contents of the Registration Statement on Form S-8 (File No.
333-24863) of Alexion Pharmaceuticals, Inc., as filed with the Securities and
Exchange Commission on April 9, 1997, are incorporated herein by reference,
except as otherwise disclosed herein, in accordance with GENERAL INSTRUCTION
E--REGISTRATION OF ADDITIONAL SECURITIES of Form S-8.




<PAGE>



                                     PART II

               INFORMATION REQUIRED IN THE REGISTRATION STATEMENT


ITEM 3.  Incorporation of Documents by Reference

         The following documents filed by Alexion Pharmaceuticals, Inc. (the
"Company") are incorporated herein by reference:

         (1)      The Company's Annual Report on Form 10-K for the fiscal year 
                  ended July 31,1998.

         (2)      The Company's Quarterly Report on Form 10-Q for the quarterly
                  period ended October 31, 1998.

         (3)      The Company's Current Reports on Form 8-K, filed on October 9,
                  1998, December 31, 1998 and January 29, 1999.

         (4)      The Company's Registration Statement on Form S-8 (File No.
                  333-24863), filed on April 9, 1997.

         (5)      The description of the Company's Common Stock contained in
                  Item 1 of the Company's Registration Statement on Form 8-A,
                  dated February 12, 1996.

         In addition to the foregoing, all documents subsequently filed by the
Company pursuant to Sections 13(a), 13(c), 14 and 15(d) of the Securities
Exchange Act of 1934, prior to the filing of a post-effective amendment
indicating that all of the securities offered hereunder have been sold or
deregistering all securities then remaining unsold, shall be deemed to be
incorporated by reference in this Registration Statement and to be part hereof
from the date of filing of such documents. Any statement contained in a document
incorporated by reference in this Registration Statement shall be deemed to be
modified or superseded for purposes of this Registration Statement to the extent
that a statement contained herein or in any subsequently filed document that is
also incorporated by reference herein modifies or supersedes such statement. Any
statement so modified or superseded shall not be deemed, except as so modified
or superseded, to constitute a part of this Registration Statement.

ITEM 8.       EXHIBITS

         4(a)     Alexion Pharmaceuticals, Inc. 1992 Stock Option Plan, as
                  amended.

         *(b)     Form of Incentive Option Agreement.

         *(c)     Form of Nonqualified Option Agreement.

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


                                      II-1


<PAGE>



         23(a)    Consent of Arthur Andersen LLP.

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

         24       Power of Attorney (included in signature page).


           *      Previously filed with the Company's Registration Statement on
                  Form S-8 (File No. 333-24863) filed on April 9, 1997.

                                      II-2


<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 5,
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.

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

<TABLE>

                                       
<S>                                 <C>                                               <C> 
/S/ LEONARD BELL                         President, Chief Executive Officer,
- -----------------------------------      Secretary, Treasurer and Director
Leonard Bell, M.D.                       (principal executive officer)                      February 5 , 1999

                                      
/S/ DAVID W. KEISER                       Executive Vice President and Chief
- -----------------------------------       Operating Officer (principal financial            February 5, 1999
David W. Keiser                           officer)

                                      
/S/ BARRY P. LUKE                         Vice President of Finance and
- -----------------------------------       Administration (principal accounting              February 5, 1999
Barry P. Luke                             officer)                      

</TABLE>


                                      II-3


<PAGE>



<TABLE>
                                       
<S>                                 <C>                                               <C> 
/S/ JOHN H. FRIED                     Chairman of the Board of Directors                    February 5, 1999
- ----------------------------------    
John H. Fried, Ph.D.


/S/ JOSEPH A. MADRI                   Director                                              February 5, 1999
- ----------------------------------
Joseph A. Madri, Ph.D., M.D.

/S/ LEONARD MARKS, JR.                Director                                              February 5, 1999
- ----------------------------------
Leonard Marks, Jr., Ph.D.

/S/ MAX LINK                          Director                                              February 5, 1999
- ----------------------------------
Max Link, Ph.D.

/S/ EILEEN M. MORE                    Director                                              February 5, 1999
- ----------------------------------
Eileen M. More

/S/ TIMOTHY F. HOWE                   Director                                              February 5, 1999
- ------------------------------------
Timothy F. Howe
</TABLE>


                                      II-4






<PAGE>



                                                                    EXHIBIT 4(a)

                          ALEXION PHARMACEUTICALS, INC.
                             1992 STOCK OPTION PLAN
                             ----------------------


                  1. PURPOSE. The purpose of the Alexion Pharmaceuticals, Inc.
1992 Stock Option Plan (the "Plan") is to enable Alexion Pharmaceuticals, Inc.
(the "Company") and its stockholders to secure the benefits of common stock
ownership by key personnel of the Company and its subsidiaries. The Board of
Directors of the Company (the "Board") believes that the granting of options
under the Plan will foster the Company's ability to attract, retain and motivate
those individuals who will be largely responsible for the profitability and
long-term future growth of the Company.

                  2. STOCK SUBJECT TO THE PLAN. The Company may issue and sell a
total of 3,100,000 shares of its common stock, $.0001 par value (the "Common
Stock"), pursuant to the Plan. Such shares may be either authorized and unissued
or held by the Company in its treasury. New options may be granted under the
Plan with respect to shares of Common Stock which are covered by the unexercised
portion of an option which has terminated or expired by its terms, by
cancellation or otherwise.

                  3. ADMINISTRATION. The Plan will be administered by a
committee (the "Committee") consisting of at least two directors appointed by
and serving
 at the pleasure of the Board. Subject to the provisions of the Plan,
the Committee, acting in its sole and absolute discretion, will have full power
and authority to grant options under the Plan, to interpret the provisions of
the Plan, to fix and interpret the provisions of option agreements made under
the Plan, to supervise the administration of the Plan, and to take such other
action as may be necessary or desirable in order to carry out the provisions of
the Plan. A majority of the members of the Committee will constitute a quorum.
The Committee may act by the vote of a majority of its members present at a
meeting at which there is a quorum or by unanimous written consent. The decision
of the Committee as to any disputed question, including questions of
construction, interpretation and administration, will be final and conclusive on
all persons. The Committee will keep a record of its proceedings and acts and
will keep or cause to be kept such books and records as may be necessary in
connection with the proper administration of the Plan.

                  4. ELIGIBILITY. Options may be granted under the Plan to
present or future key employees of the Company or a subsidiary of the Company (a
"Subsidiary") within the meaning of Section 424(f) of the Internal Revenue Code
of 1986 (the "Code"), and to consultants to the Company or a Subsidiary who are
not employees. Options may be granted to directors of the Company or a
Subsidiary whether or not also employees of or consultants to the Company and/or
a Subsidiary. Subject to the provisions of the Plan, the Committee may from time
to time select the persons to whom options will be granted, and will fix the
number of shares covered by each such option and establish the terms and
conditions thereof (including, without limitation, exercise price and
restrictions on exercisability of the option or on the shares of Common Stock
issued upon exercise thereof and whether or not the option is to be treated as
an incentive stock option within the meaning of Section 422 of the Code (an
"Incentive Stock Option").




<PAGE>



                  5. TERMS AND CONDITIONS OF OPTIONS. Each option granted under
the Plan will be evidenced by a written agreement in a form approved by the
Committee. Each such option will be subject to the terms and conditions set
forth in this paragraph and such additional terms and conditions not
inconsistent with the Plan (and, in the case of an Incentive Stock Option, not
inconsistent with the provisions of the Code applicable thereto) as the
Committee deems appropriate.

                           (1) OPTION EXERCISE PRICE. In the case of an option
which is not treated as an Incentive Stock Option, the exercise price per share
may not be less than the par value of a share of Common Stock on the date the
option is granted; and, in the case of an Incentive Stock Option, the exercise
price per share may not be less than 100% of the fair market value of a share of
Common Stock on the date the option is granted (110% in the case of an optionee
who, at the time the option is granted, owns stock possessing more than 10% of
the total combined voting power of all classes of stock of the Company or a
Subsidiary (a "ten percent shareholder")). For purposes hereof, the fair market
value of a share of Common Stock on any date will be equal to the closing sale
price per share as published by a national securities exchange on which shares
of the Common Stock are traded on such date or, if there is no sale of Common
Stock on such date, the average of the bid and asked prices on such exchange at
the closing of trading on such date or, if shares of the Common Stock are not
listed on a national securities exchange on such date, the closing price or, if
none, the average of the bid and asked prices in the over the counter market at
the close of trading on such date, or if the Common Stock is not traded on a
national securities exchange or the over the counter market, the fair market
value of a share of the Common Stock on such date as determined in good faith by
the Committee.

                           (2) OPTION PERIOD. The period during which an option
may be exercised will be fixed by the Committee and will not exceed 10 years
from the date the option is granted (5 years in the case of an Incentive Stock
Option granted to a "ten percent shareholder").

                           (3) EXERCISE OF OPTIONS. Unless otherwise determined
by the Committee, no option will become exercisable unless the person to whom
the option was granted remains in the continuous employ or service of the
Company or a Subsidiary for at least one year (or for such other period as the
Committee may designate) from the date the option is granted. Subject to earlier
termination of the option as provided herein, unless the Committee determines
otherwise, 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 date of
grant:

                                       -2-


<PAGE>


<TABLE>
<CAPTION>
        Full                        Incremental              Cumulative
       Years of Continuous          Percentage of            Percentage of
       Employment/                  Option                   Option
       Service                      Exercisable              Exercisable  
       -------                      -----------              -----------  
       <S>                          <C>                      <C>
       Less than 1                       0%                       0%
                 1                      25%                      25%
                 2                      25%                      50%
                 3                      25%                      75%
                 4 or more              25%                     100%
</TABLE>


All or part of the exercisable portion of an option may be exercised at any time
during the option period, except that, without the consent of the Committee, no
partial exercise of an option may be for less than 100 shares. An option may be
exercised by transmitting to the Company (1) a written notice specifying the
number of shares to be purchased, and (2) payment of the exercise price (or, if
applicable, delivery of a secured obligation therefor), together with the
amount, if any, deemed necessary by the Committee to enable the Company to
satisfy its income tax withholding obligations with respect to such exercise
(unless other arrangements acceptable to the Company are made with respect to
the satisfaction of such withholding obligations).

                           (4) PAYMENT OF EXERCISE PRICE. The purchase price of
shares of Common Stock acquired pursuant to the exercise of an option granted
under the Plan may be paid in cash and/or such other form of payment as may be
permitted under the option agreement, including, without limitation,
previously-owned shares of Common Stock. The Committee may permit the payment of
all or a portion of the purchase price in installments (together with interest)
over a period of not more than 5 years.

                           (5) RIGHTS AS A STOCKHOLDER. No shares of Common
Stock will be issued in respect of the exercise of an option granted under the
Plan until full payment therefor has been made (and/or provided for where all or
a portion of the purchase price is being paid in installments). The holder of an
option will have no rights as a stockholder with respect to any shares covered
by an option until the date a stock certificate for such shares is issued to him
or her. Except as otherwise provided herein, no adjustments 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.

                           (6) NONTRANSFERABILITY OF OPTIONS. No option shall be
assignable or transferrable except upon the optionee's death to a beneficiary
designated by the optionee in accordance with procedures established by the
Committee or, if no designated beneficiary shall survive the optionee, pursuant
to the optionee's will or by the laws of descent and distribution. During an
optionee's lifetime, options may be exercised only by the optionee or the
optionee's guardian or legal representative.

                           (7) TERMINATION OF EMPLOYMENT OR OTHER SERVICE. If an
optionee ceases to be employed by or to perform services for the Company and any
Subsidiary for any reason other than death or disability (defined below), then,
unless the Committee determines otherwise, each outstanding option granted to
him or her under the Plan will terminate on the date three months after

                                      -3-

<PAGE>

the date of such termination of employment or service. If an 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 each outstanding option granted to
the optionee under the Plan will terminate on the date one year after the date
of such termination of employment or service (or one year after the later death
of a disabled optionee) or, if earlier, the date specified in the option
agreement. For purposes hereof, the term "disability" means the inability of an
optionee to perform the customary duties of his or her employment or other
service for the Company or a Subsidiary by reason of a physical or mental
incapacity which is expected to result in death or be of indefinite duration.



                           (8) MAXIMUM OPTION GRANT. The maximum option grant
which may be made to an employee of the Company in any calendar year shall not
cover more than 200,000 shares.

                           (9) OTHER PROVISIONS. The Committee may impose such
other conditions with respect to the exercise of options, including, without
limitation, any conditions relating to the application of federal or state
securities laws, as it may deem necessary or advisable.

                  6.       CAPITAL CHANGES, REORGANIZATION, SALE.

                           (1) ADJUSTMENTS UPON CHANGES IN CAPITALIZATION. The
aggregate number and class of shares for which options may be granted under the
Plan, the number and class of shares covered by each outstanding option and the
exercise price per share shall all be adjusted proportionately for any increase
or decrease in the number of issued shares of Common Stock resulting from a
split-up or consolidation of shares or any like capital adjustment, or the
payment of any stock dividend.

                           (2) CASH, STOCK OR OTHER PROPERTY FOR STOCK. 
Except as otherwise provided in this subparagraph, in the event of an 
Exchange Transaction (as defined below), all optionees will be permitted to 
exercise their outstanding options (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.

                           (3) DEFINITION OF EXCHANGE TRANSACTION. For purposes
hereof, the term "Exchange Transaction" means a 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, acquisition 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 Shareholders of the Company receive cash, stock or other property
in exchange for or in connection with their


                                      -4-

<PAGE>

shares of Common Stock.

                           (4) FRACTIONAL SHARES. 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.

                           (5) DETERMINATION OF BOARD TO BE FINAL. All
adjustments under this paragraph 6 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. Unless an optionee agrees otherwise, any
change or adjustment to an Incentive Stock Option shall be made in such a manner
so as not to constitute a "modification" as defined in Section 424(h) of the
Code and so as not to cause the hereunder to fail to continue to qualify as an
Incentive Stock Option.

                  7. AMENDMENT AND TERMINATION OF THE PLAN. The Board may amend
or terminate the Plan. Except as otherwise provided in the Plan with respect to
equity changes, any amendment which would increase the aggregate number of
shares of Common Stock as to which options may be granted under the Plan,
materially increase the benefits under the Plan, or modify the class of persons
eligible to receive options under the Plan shall be subject to the approval of
the holders of a majority of the Common Stock issued and outstanding. No
amendment or termination may affect adversely any outstanding option without the
written consent of the optionee.

                  8. NO RIGHTS CONFERRED. Nothing contained herein will be
deemed to give any individual any right to receive an option under the Plan or
to be retained in the employ or service of the Company or any Subsidiary.

                  9. GOVERNING LAW. The Plan and each option agreement shall be
governed by the laws of the State of Delaware.

                  10. DECISIONS AND DETERMINATIONS OF COMMITTEE TO BE FINAL.
Except to the extent rights or powers under this Plan are reserved specifically
to the discretion of the Board, all decisions and determinations of the
Committee are final and binding.

                  11. TERM OF THE PLAN. The Plan shall be effective as of
February 27, 1992, the date on which it was adopted by the Board, subject to the
approval of the stockholders of the Company, which approval was granted on
February 27, 1992. The Plan will terminate on February 26, 2002, the date ten
years after the date of adoption by the Board, unless sooner terminated by the
Board. The rights of optionees under the options outstanding at the time of the
termination of the Plan shall not be affected solely by reason of the
termination and shall continue in accordance with the terms of the option (as
then in effect or thereafter amended).


                                       -5-





<PAGE>

                               [LETTERHEAD]





                                                                       EXHIBIT 5




                                              February 5, 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 the additional 1,300,000
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, 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(a)


                    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