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                                                                     Exhibit 5.1

                                 August 30, 2000



Microcide Pharmaceuticals, Inc.
850 Maude Avenue
Mountain View, California 94043


         RE: REGISTRATION STATEMENT ON FORM S-8


Ladies and Gentlemen:

         We have examined the Registration Statement on Form S-8 to be filed by
you with the Securities and Exchange Commission on or about August 30, 2000 (the
"Registration Statement") in connection with the registration under the
Securities Act of 1933, as amended, for an aggregate of 450,000 shares of your
Common Stock under the 1993 Amended Incentive Stock Plan. Such shares of Common
Stock are referred to herein as the "Shares", and such plan is referred to
herein as the "Plan". As your counsel in connection with this transaction, we
have examined the proceedings taken and are familiar with the proceedings
proposed to be taken by you in connection with the issuance and sale of the
Shares pursuant to the Plan.

         It is our opinion that, when issued and sold in the manner described in
the Plan and pursuant to the agreements which accompany each grant under the
Plan, the Shares will be legally and validly issued, fully-paid and
non-assessable.

         We consent to the use of this opinion as an exhibit to the Registration
Statement, and further consent to the use of our name wherever appearing in the
Registration Statement and any amendments thereto.


                                       Very truly yours,

                                       WILSON SONSINI GOODRICH & ROSATI
                                       Professional Corporation



                                       /s/ Wilson Sonsini Goodrich & Rosati



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