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

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                                  SCHEDULE 13G

                    Under the Securities Exchange Act of 1934


                          ALEXION PHARMACEUTICALS, INC.
                                (Name of Issuer)

                                  Common Stock
                         (Title of Class of Securities)

                                    015351109
                                 (CUSIP Number)



















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Check the following box if a
fee is being paid with this statement.  |_|


2 CUSIP No. 015351109 (1) Name of Reporting Person S.S. or I.R.S. Identification No. of Above Person Citibank, N.A.; 13-5266470 ---------------------------------------------------------------- ---------------------------------------------------------------- (2) Check the Appropriate Box if a Member of Group (See Instructions) |_| (a) ------------------------------------------------------------- |_| (b) ------------------------------------------------------------- (3) SEC Use Only --------------------------------------------------- ---------------------------------------------------------------- (4) Citizenship or Place of Organization New York -------------------------- Number of (5) Sole Voting Power 0 Shares --------------------------------- Beneficially --------------------------------------------------- Owned by (6) Shared Voting Power 417,575 Each ------------------------------- Reporting --------------------------------------------------- Person (7) Sole Dispositive Power 0 With -------------------------- - ------ --------------------------------------------------- (8) Shared Dispositive Power 417,575 -------------------------- --------------------------------------------------- --------------------------------------------------- (9) Aggregate Amount Beneficially Owned by Each Reporting Person 417,575 -------------------------------------- (10) Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) ---------------------------------------------------------------- ---------------------------------------------------------------- (11) Percent of Class Represented by Amount in Row (9) 3.7 % ---------------------------------------------------------------- (12) Type of Reporting Person (See Instructions) BK* -------------------- - -------- * The reporting person is a pledgee with limited voting and dispositive power pursuant to certain voting trust agreements. The reporting person disclaims beneficial ownership of these securities, and this report shall not be deemed an admission that the reporting person is the beneficial owner of such securities for purposes of Section 13 or for any other purpose.

3 Item 1. Issuer (a) Alexion Pharmaceuticals, Inc. (b) 25 Science Park New Haven, CT 06511 Item 2. Persons Filing (a) Name of Persons Filing: Citibank, N.A. (b) Address of Principal Offices: 399 Park Avenue, New York, New York 10043 (Citibank, N.A.) (c) Citizenship: New York (Citibank, N.A.) (d) Title of Class of Securities: Common Stock (e) CUSIP Number: 015351109 Item 3. If this Statement is filed pursuant to Rules 13d-1(b) or 13d-2(b), check whether the person filing is a: (a) |_| Broker or Dealer registered under Section 15 of the Act (b) |X| Bank as defined in Section 3(a)(6) of the Act (c) |_| Insurance Company as defined in Section 3(a) (19) of the Act (d) |_| Investment Company registered under Section 8 of the Investment Company Act (e) |_| Investment Adviser registered under Section 203 of the Investment Advisers Act of 1940 (f) |_| Employee Benefit Plan, Pension Fund which is subject to the provisions of the Employee Retirement Income Security Act of 1974 or Endowment Fund; see ss. 240.13d-1(b)(1)(ii)(F) (g) |_| Parent Holding Company, in accordance with ss. 240.13d-1(b)(ii)(G) (h) |_| Group in accordance with ss. 240.13d-1(b)(1)(ii)(G) Item 4. Ownership (a) Citibank, N.A.: 417,575; (b) Citibank, N.A.: 3.7%; (c) (i) -0-; (ii) Citibank, N.A.: 417,575; (iii) -0-; (iv) Citibank, N.A.: 417,575.

4 Item 5. Ownership of Five Percent or Less of a Class This statement is being filed to report that as of the date hereof the reporting person has ceased to be the beneficial owner of more than five percent of the class of securities. | | Item 6. Ownership of More than Five Percent on Behalf of Another Person Not applicable. Item 7. Identification and Classification of the Subsidiary Which Acquired the Security Being Reported on by the Parent Holding Company Not applicable. Item 8. Identification and Classification of Members of the Group Not Applicable. Item 9. Notice of Dissolution of Group Not Applicable. Item 10. Certification By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect.

5 SIGNATURE After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct. Date: February 16, 1999 CITIBANK, N.A. By: /s/ Melissa Tanklefsky --------------------------------- Name: Melissa Tanklefsky Title: Vice President