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                                                                     Exhibit 5.1





                               September 1, 1999


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 September 1, 1999
(the "Registration Statement") in connection with the registration under the
Securities Act of 1933, as amended, for an aggregate of 470,000 shares of your
Common Stock under the 1993 Amended Incentive Stock Plan, 1996 Director Option
Plan and 1996 Employee Stock Purchase 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



                                        /s/ Wilson Sonsini Goodrich & Rosati