Exhibit 5.1


                                 June 27, 2003

Genome Therapeutics Corp.
100 Beaver Street
Waltham, Massachusetts  02453

         Re:  Genome Therapeutics Corp.
              -------------------------

Ladies and Gentlemen:

         This opinion is furnished to you in connection with a registration
statement on Form S-3 (the "Registration Statement"), filed today with the
Securities and Exchange Commission under the Securities Act of 1933, as amended,
for the registration of 486,646 shares of common stock, $0.10 par value (the
"Shares"), of Genome Therapeutics Corp., a Massachusetts corporation (the
"Company") issuable upon exercise of warrants to purchase 486,646 shares of the
Company's common stock (the "Warrants"). The Shares issuable upon exercise of
the Warrants are referred to herein as the "Warrant Shares". The Company
originally issued the Warrants pursuant to an Amendment, Redemption and Exchange
Agreement dated as of June 4, 2003, among the Company, Smithfield Fiduciary LLC
and The Tail Wind Fund, Ltd. The Shares are being registered to permit the
secondary trading of such Shares by the holders thereof from time to time after
the effective date of the Registration Statement.

         We have acted as counsel for the Company in connection with its
issuance and sale of the Warrants and the preparation of the Registration
Statement. For purposes of this opinion, we have examined and relied upon a
signed copy of the Registration Statement and such other documents, records,
certificates and instruments as we have deemed necessary.

         The opinions expressed below are limited to matters governed by the
laws of the Commonwealth of Massachusetts and the federal laws of the United
States of America.

         Based on the foregoing, we are of the opinion that the Shares have been
duly authorized and, when the Shares have been issued in accordance with the
terms of the Warrants, including without limitation when the Company has
received the consideration in accordance with the terms of Warrants, the Shares
will be validly issued, fully paid and non-assessable.



         We hereby consent to your filing this opinion as an exhibit to the
Registration Statement and to the use of our name therein and in the related
prospectus under the caption "Legal Matters".

         It is understood that this opinion is to be used only in connection
with the offer and sale of the Shares while the Registration Statement is in
effect.

                                                     Very truly yours,

                                                     /s/ Ropes & Gray

                                                     Ropes & Gray







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