Exhibit 5.1


                               December 19, 2000




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 December 19, 2000, in connection with the
registration under the Securities Act of 1933, as amended, of an aggregate of
2,415,290 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