EXHIBIT 5.1
 
   
                               September 22, 1998
    
 
ProBusiness Services, Inc.
4125 Hopyard Road
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 August 5, 1998 (Registration No.
333-60745), as amended (the "Registration Statement"), in connection with the
registration under the Securities Act of 1933, as amended, of up to 2,846,250
shares of your Common Stock (the "Shares"), including an over-allotment option
granted to the underwriters of the offering to purchase up to an additional
371,250 shares. We understand that you are selling the Shares to the
underwriters 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.
    
 
    It is our opinion that, upon completion of the proceedings being taken or
proposed to be taken by us, as your legal counsel, prior to the issuance of the
Shares, the Shares will be legally issued, fully paid and non-assessable when
sold in the manner described in the Registration Statement.
 
    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