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                                                                       EXHIBIT 5
 
                                                                 January 9, 1996
 
Genome Therapeutics Corp.
100 Beaver Street
Waltham, Massachusetts 02154
 
  Re:  Registration Statement on
       Form S-3
 
Ladies and Gentlemen:
 
     This opinion is rendered to you in connection with the above-mentioned
registration statement (the "Registration Statement") on Form S-3 filed with the
Securities and Exchange Commission under the Securities Act of 1933, as amended
(the "Act"), for the registration under the Act by Genome Therapeutics Corp.
(the "Company") of up to 2,300,000 shares of the Company's Common Stock, $0.10
par value per share (the "Shares").
 
     The Shares are to be sold pursuant to an underwriting agreement (the
"Underwriting Agreement") to be entered into between the Company and the
underwriter named therein (the "Underwriter").
 
     We have acted as counsel to the Company in connection with the issuance and
sale of the Shares and the preparation of the Registration Statement. For
purposes of this opinion, we have examined and relied upon such documents and
records as we have deemed necessary.
 
     We have assumed that the Underwriting Agreement will have been duly
executed and delivered, all in accordance with the authorizing resolutions of
the Board of Directors of the Company, and that the Shares will be sold and
transferred only upon payment therefor and as provided in the Underwriting
Agreement.
 
     Based upon the foregoing, we are of the opinion that the Shares have been
duly authorized and when issued, delivered, and paid for as provided in the
Underwriting Agreement will be validly issued, fully-paid, and nonassessable.
 
     We hereby consent to the filing of this opinion as part of 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