Exhibit 5.1






                                                                 July 9, 1998


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 10,  1998 (the
"Registration   Statement")  in  connection  with  the  registration  under  the
Securities  Act of 1933, as amended,  for an aggregate of 400,000 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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