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                                                                     EXHIBIT 5.1


                                 June 19, 2001

ProBusiness Services, Inc.
4025 Hopyard Road
Pleasanton, CA  94588

         RE: REGISTRATION STATEMENT ON FORM S-8

Ladies and Gentlemen:

         We have examined the Registration Statement on Form S-8 (the
"Registration Statement") to be filed by ProBusiness Services, Inc., a Delaware
corporation (the "Registrant" or "you"), with the Securities and Exchange
Commission on or about June , 2001, in connection with the registration under
the Securities Act of 1933, as amended, of an aggregate of 498,530 shares of
your Common Stock, $0.001 par value (the "Shares"), reserved for issuance
pursuant to the 1996 Stock Option Plan (the "Plan"). 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 by the Registrant under the Plan.

         It is our opinion that the Shares will be, when issued and sold in the
manner referred to in the Plan, legally 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,



                                     /s/ WILSON SONSINI GOODRICH & ROSATI
                                     Professional Corporation