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                [WILSON, SONSINI, GOODRICH & ROSATI LETTERHEAD]

                                                                    Exhibit 5.1



                               September 11, 1996


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

     RE:  Registration Statement on Form S-8

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 September 11, 1996 (the
"Registration Statement") in connection with the registration under the
Securities Act of 1933, as amended, for an aggregate of 1,580,000 shares of your
Common Stock under the 1993 Amended Incentive Stock Plan, the 1996 Employee
Stock Purchase Plan, and the 1996 Director Option Plan. Such shares of Common
Stock are referred to herein as the "Shares", and such plans are referred to
herein as the "Plans". 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 Plans.

     It is our opinion that, when issued and sold in the manner described in the
Plans and pursuant to the agreements which accompany each grant under the Plans,
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