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