EXHIBIT 5.1

                [Letterhead of Wilson Sonsini Goodrich & Rosati]

                               September 30, 1999

ProBusiness Services, Inc.
4125 Hopyard Road
Pleasanton, CA 94588

    Re:  Registration Statement on Form S-3

Ladies and Gentlemen:

    We have examined the Registration Statement on Form S-3 to be filed by you
with the Securities and Exchange Commission on or about September 30, 1999 (the
"Registration Statement"), in connection with the registration under the
Securities Act of 1933, as amended, of up to 1,750,967 shares of your Common
Stock (the "Shares"). As your legal counsel, we have examined the proceedings
taken by you in connection with the issuance and sale of the Shares.

    It is our opinion that the Shares have been legally issued and are 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, including the Prospectus constituting a part thereof,
and any amendments thereto.

                                          Very truly yours,
                                          WILSON SONSINI GOODRICH & ROSATI
                                          Professional Corporation
                                          /s/ Wilson Sonsini Goodrich & Rosati