Exhibit 5


                            [ROPES & GRAY LETTERHEAD]



                                                     March 15, 2000



Genome Therapeutics Corp.
100 Beaver Street
Waltham, MA  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 with the
Securities and Exchange Commission under the Securities Act of 1933, as
amended, for the registration of 3,450,000 shares of Common Stock, $.10 par
value (the "Shares"), of Genome Therapeutics Corp., a Massachusetts
corporation (the "Company"). The Shares are to be sold pursuant to an
underwriting agreement (the "Underwriting Agreement") to be entered into
among the Company and CIBC World Markets, as representatives of the
underwriters named therein.

         We have acted as counsel for the Company in connection with its
proposed issuance and sale of the Shares. For purposes of this opinion, we have
examined and relied upon such documents, records, certificates and other
instruments as we have deemed necessary.

         We express no opinion as to the applicability or compliance with or
effect of Federal law or the law of any jurisdiction other than The Commonwealth
of Massachusetts and the corporate laws of The Commonwealth of Massachusetts.

         Based on the foregoing, we are of the opinion that the shares have been
duly authorized and, when the Shares have been issued and sold and the Company
has received the consideration in accordance with the terms of the Underwriting
Agreement, the Shares will be validly issued, fully paid and non-assessable.





Name of Company                        -2-                                [date]


         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 "Validity of Common Stock."

         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