Exhibit 5.1


                                  April 9, 2002



ProBusiness Services, Inc.
4125 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 April 9, 2002, in connection with the registration under
the Securities Act of 1933, as amended, of an aggregate of 2,675,546 shares of
your Common Stock, $0.001 par value (the "Shares"), reserved for issuance
pursuant to the 1996 Stock Option Plan and the 1997 Employee Stock Purchase Plan
(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 by the Registrant 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 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