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                                                                    EXHIBIT 5.1



                               October 3, 1997



ProBusiness Services, Inc.
5934 Gibraltar Drive
Pleasanton, California 94588

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 October 3, 1997 (the
"Registration Statement"), in connection with the registration under the
Securities Act of 1933, as amended, of an aggregate of 2,694,912 shares of
Common Stock, par value $0.001 (the "Shares"), reserved for issuance pursuant
to the 1989 Stock Option Plan, the 1996 Stock Option Plan and the 1997 Employee
Stock Purchase Plan (together, the "Plans"). As your legal counsel, we have
examined the proceedings taken and are familiar with the proceedings proposed
to be taken by you in connection with the sale and issuance of the shares under
the Plans.

     It is our opinion that the Shares will be, when issued and sold in the
manner referred to in the Plans, legally and validly issued, fully paid and
nonassessable.

     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 subsequent amendment thereto.

                                            Very truly yours,

                                            WILSON SONSINI GOODRICH & ROSATI
                                            Professional Corporation


                                            /s/ Wilson Sonsini Goodrich & Rosati