EXHIBIT 5

                            [Ropes & Gray Letterhead]

July 26, 2002

Genome Therapeutics Corp.
100 Beaver Street
Waltham, Massachusetts  02453

Ladies and Gentlemen:

We have acted as counsel for Genome Therapeutics Corp., a Massachusetts
corporation (the "Company"), in connection with the preparation of a
registration statement on Form S-8 and all exhibits thereto (the "Registration
Statement") filed with the Securities and Exchange Commission under the
Securities Act of 1933, as amended, for the registration of 250,000 shares of
Common Stock, $.10 par value (the "Shares").

The Shares being registered are issuable pursuant to the Company's Employee
Stock Purchase Plan, as amended through June 25, 2002 (the "Plan"), which is
attached as an exhibit to the Registration Statement.

For purposes of this opinion, we have examined a copy of the Registration
Statement; a copy of the Plan; the Restated Articles of Organization of the
Company, as amended to date; the By-laws of the Company, as amended to date; the
votes of the Board of Directors of the Company adopting the Plan; and such other
documents and records as we deem necessary for purposes of this opinion.

We have assumed that the Shares will be issued only as provided in the Plan and
that the price at which such Shares are issued will not be less than the par
value per share of the Company's Common Stock. We have also assumed that the
issuance of any such shares will not result in the issuance by the Company of
more than its authorized shares of Common Stock.

Based upon and subject to the foregoing, we are of the opinion that the Shares
have been duly authorized and that the Shares, when issued in accordance with
the terms of the Plan, will have been validly issued and will be fully paid and
nonassessable.

In connection with any issue and sale of the Shares, steps should be taken to
effect compliance with all applicable laws, rules and regulations of
governmental authorities regulating sales and offerings of securities.

We understand that this opinion is to be used in connection with the
Registration Statement. We consent to the filing of this opinion as an exhibit
to the Registration Statement and to the use of our name therein.

Very truly yours,

/s/ ROPES & GRAY

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