1
                                                                     Exhibit 5.1
                                                                     -----------






                                 August 4, 1997


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 July 31,
1997 (the "Registration Statement") in connection with the registration under
the Securities Act of 1933, as amended, for an aggregate of 500,000 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