[Golenbock, Eiseman, Assor & Bell Letterhead]

                                                                       Exhibit 5

                                                               December 28, 2000

Alexion Pharmaceuticals, Inc.
352 Knotter Drive
Cheshire, Connecticut 06410

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 44,292 shares of
the Company's Common Stock, $.0001 par value per share (the "Shares"), to be
issued under the Prolifarion, Inc. 1999 Long Term Incentive And Stock Option
Plan (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/ Golenbock, Eiseman, Assor & Bell


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