1
                                                                     EXHIBIT 5.1

                                 SEPTEMBER 18, 1997
PROBUSINESS SERVICES, INC.        
5934 Gibraltar Drive
Pleasanton, CA 94588


         RE:      REGISTRATION STATEMENT ON FORM S-1

Ladies and Gentlemen:

        We have examined the Registration Statement on Form S-1 filed by you
with the Securities and Exchange Commission on March 12, 1997 (Registration No.
333-23189), as amended (the "Registration Statement"), in connection with the
registration under the Securities Act of 1933, as amended, of up to 2,400,000
shares of your Common Stock, $0.001 par value per share (the "Shares"). The
Shares include an over-allotment option granted to the underwriters of the
offering to purchase up to 360,000 shares. We understand that the Shares are to
be sold to the underwriters of the offering for resale to the public  as
described in the Registration Statement. 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 to
be sold by you.

         It is our opinion that upon completion of the proceedings being
taken or contemplated by us, as your counsel, to be taken prior to the issuance
of the Shares, including the proceedings being taken in order to permit such
transaction to be carried out in accordance with applicable state securities
laws, the Shares, when issued and sold in the manner described in the
Registration Statement, will be legally issued, fully paid and non-assessable.

         We are members of the Bar of the State of California only and express
no opinion as to any matter relating to the laws of any jurisdiction other than
the laws of the State of California and the federal laws of the United States.
Without limiting the foregoing, we express no opinion as to the securities laws
of the State of Delaware.

         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