SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549

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

                                    FORM 8-K

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

                                 CURRENT REPORT
                     PURSUANT TO SECTION 13 OR 15(d) OF THE
                         SECURITIES EXCHANGE ACT OF 1934

Date of Report (Date of earliest event reported) February 25, 2000
                                                 -----------------

                         ALEXION PHARMACEUTICALS, INC.
-------------------------------------------------------------------------------
             (Exact Name of Registrant as Specified in its Charter)

            Delaware                   0 -27756               13-3648318
----------------------------         -----------          ------------------
(State or Other Jurisdiction         (Commission            (IRS Employer
         of Incorporation)            File Number)        Identification No.)

           25 Science Park, New Haven, CT                       06511
       ---------------------------------------               ---------
       (Address of Principal Executive Offices)              (Zip Code)

Registrant's telephone number, including area code: (203) 776-1790
                                                   -----------------

                                 Not Applicable
--------------------------------------------------------------------------------
          (Former Name or Former Address, if Changed Since Last Report)


<PAGE>

Item 5. Other Events

      On February 25, 2000, Alexion Pharmaceuticals, Inc. issued the press
release filed herewith as Exhibit 99.


Item 7. Financial Statements, Pro Forma Financial Information and Exhibits.

      (c) Exhibits.

      99     Press Release dated February 25, 2000.


<PAGE>

                                   SIGNATURES


      Pursuant to the requirements of the Securities Exchange Act of 1934, the
registrant has duly caused this report to be signed on its behalf by the
undersigned hereunto duly authorized.

                               ALEXION PHARMACEUTICALS, INC.


Date: February 24, 2000        By:    /s/ Leonard Bell
                                  ---------------------------------------
                               Name:  Leonard Bell, M.D.
                               Title: President, Chief Executive Officer,
                                      Secretary and Treasurer
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SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549

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

                                    FORM 8-K

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

                                 CURRENT REPORT
                     PURSUANT TO SECTION 13 OR 15(d) OF THE
                         SECURITIES EXCHANGE ACT OF 1934

Date of Report (Date of earliest event reported) February 25, 2000
                                                 -----------------

                         ALEXION PHARMACEUTICALS, INC.
-------------------------------------------------------------------------------
             (Exact Name of Registrant as Specified in its Charter)

            Delaware                   0 -27756               13-3648318
----------------------------         -----------          ------------------
(State or Other Jurisdiction         (Commission            (IRS Employer
         of Incorporation)            File Number)        Identification No.)

           25 Science Park, New Haven, CT                       06511
       ---------------------------------------               ---------
       (Address of Principal Executive Offices)              (Zip Code)

Registrant's telephone number, including area code: (203) 776-1790
                                                   -----------------

                                 Not Applicable
--------------------------------------------------------------------------------
          (Former Name or Former Address, if Changed Since Last Report)
Item 5. Other Events

      On February 25, 2000, Alexion Pharmaceuticals, Inc. issued the press
release filed herewith as Exhibit 99.

Item 7. Financial Statements, Pro Forma Financial Information and Exhibits.

      (c) Exhibits.

      99     Press Release dated February 25, 2000.
SIGNATURES

      Pursuant to the requirements of the Securities Exchange Act of 1934, the
registrant has duly caused this report to be signed on its behalf by the
undersigned hereunto duly authorized.

                               ALEXION PHARMACEUTICALS, INC.

Date: February 24, 2000        By:    /s/ Leonard Bell
                                  ---------------------------------------
                               Name:  Leonard Bell, M.D.
                               Title: President, Chief Executive Officer,
                                      Secretary and Treasurer