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                                                                Exhibit 4.2(a)


                           PROBUSINESS SERVICES, INC.

         AMENDMENT TO AMENDED AND RESTATED REGISTRATION RIGHTS AGREEMENT

         The undersigned, being parties to that certain Amended and Restated
Registration Rights Agreement (the "Agreement") dated March 12, 1997, by and
among ProBusiness, Inc., a California corporation (the "Company"), General
Atlantic Partners 39, L.P. ("GAP L.P.") and GAP Coinvestment Partners, L.P.
("GAP Coinvestment") (collectively, the "Purchasers"), and the Holders (as
defined in the Registration Rights Agreement, dated December 1, 1989, as
amended, between the Company and the Original Holders) (the "Original
Holders"), in consideration of valuable consideration, the receipt and
sufficiency of which is hereby acknowledged, hereby agree and consent to the
following amendments to the Agreement:

         1. General Atlantic Partners 70, L.P., GAP Coinvestment Partners II,
L.P., and GapStar, LLC, shall each have the rights and obligations of a
"Purchaser," as defined in the Agreement.

         2. Section 1.2 "REGISTRABLE SECURITIES" shall be deleted and
replaced with the following Section 1.2 "REGISTRABLE SECURITIES":

         "REGISTRABLE SECURITIES" means (i) shares of the Company's Common
Stock issued or issuable pursuant to the conversion of the Company's
Preferred Stock, (ii) any Common Stock of the Company issued or issuable in
respect of the shares of the Company's Common Stock or other securities
issued or issuable pursuant to the conversion of the Company's Preferred
Stock upon any stock split, stock dividend, recapitalization or similar
event, (iii) shares of the Company's Common Stock issued or issuable pursuant
to the conversion of the Company's 6.9% Senior Convertible Preferred Stock
issued on August 1, 2000 (the "Senior Preferred Stock"), and (iv) any Common
Stock of the Company issued or issuable in respect of the shares of the
Company's Common Stock or other securities issued or issuable pursuant to the
conversion of the Senior Preferred Stock upon any stock split, stock
dividend, recapitalization or similar event; provided, however, Registrable
Securities shall not include any such shares that were transferred by a
Holder to a transferree that did not comply with Section 1.15 hereof or of
the Original Agreement."

         3. The following provision shall be added to the Agreement below
Section 1.7.2 as Section 1.7.3:

            "1.7.3      SHELF REGISTRATION

                        (a) FILING AND EFFECTIVENESS OF SHELF REGISTRATION.
As soon as practicable following the conversion of the Senior Preferred Stock
into shares of Common

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Stock (the "CONVERSION SHARES") the Company shall file a shelf registration
statement solely with respect to the Conversion Shares and pursuant to Rule
415 under the Securities Act (the "SHELF REGISTRATION") on Form S-3 (or any
successor form). The Company shall use its reasonably commercial best efforts
to have the Shelf Registration declared effective as soon as practicable
after such filing, and shall use its best reasonably commercial efforts to
keep the Shelf Registration effective and updated, from the date such Shelf
Registration is declared effective (the "EFFECTIVE DATE") until the second
anniversary of the Effective Date (the "EFFECTIVE PERIOD"); PROVIDED, that if
the Company declares a Blackout Period (as defined below) during the
Effective Period, the Effective Period shall be extended by the number of
days equal to the length of time that any Blackout Periods were in effect.
Notwithstanding the foregoing, if the Board of Directors makes a good faith
determination that a filing of the Shelf Registration or the sale of any
Conversion Shares under an effective Shelf Registration would interfere with
any material financing or material investment transaction, business
combination or material acquisition then under consideration, involving the
Company or any of its affiliates, and the Company provides written notice
(the "SHELF NOTICE") to the holders containing a general statement of the
reasons for such determination (which shall be kept confidential by such
holders), the Company may postpone the filing of the Shelf Registration for
the period indicated in the Shelf Notice (which shall be kept confidential by
such holders), which period shall in no event exceed 90 days (a "BLACKOUT
PERIOD").

                        (b) SUPPLEMENTS AND AMENDMENTS; EXPENSES. The Company
shall supplement or amend, if necessary, the Shelf Registration, as required
by the instructions applicable to such registration form or by the Securities
Act or as reasonably required by the holders of (or any underwriter for) more
than 50% of the Conversion Shares and the Company shall furnish to the
holders of the Conversion Shares to which the Shelf Registration relates
copies of any such supplement or amendment prior to its being used and/or
filed with the Commission. The Company shall pay all Registration Expenses in
connection with the Shelf Registration, whether or not it becomes effective,
and whether all, none or some of the Conversion Shares are sold pursuant to
the Shelf Registration.

                        (c) EFFECTIVE SHELF REGISTRATION STATEMENT. A Shelf
Registration pursuant to this Section 1.7.3 shall not be deemed to have been
effected (i) unless a Shelf Registration has become effective and remained
effective in compliance with the provisions of the Securities Act with
respect to the disposition of all Conversion Shares and until such time as
all of such Conversion Shares have been disposed of under the Shelf
Registration or (ii) if after it has become effective, the Shelf Registration
is interfered with by any stop order, injunction or other order or
requirement of the Commission or other governmental agency or court and has
not thereafter become effective."

         4. Section 1.11.2 under the heading "LOCK-UP AGREEMENT" shall be
deleted and replaced with the following Section 1.11.2:

         "In consideration for the Company agreeing to its obligations under
this Section 1, each Purchaser and each transferee of such Purchaser pursuant
to Section 1.15 hereof agrees

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(but only if each officer, director, shareholder owning beneficially ten
percent (10%) or more of the Company's equity securities, and each
shareholder selling shares in such offering also agrees), in connection with
the first registration (an "Initial Public Offering") of the Company's
securities for its own account to be offered to the general public (other
than a registration relating to a Rule 145 transaction or with respect to an
employee benefit plan) and in connection with any subsequent registration of
the Company's securities that occurs within the five-year period after the
closing of an Initial Public Offering, not to sell, make any short sale of,
loan, grant any option for the purchase of, or otherwise dispose of any
Registrable Securities or other securities of the Company (other than those
included in the registration) without the prior written consent of the
underwriters, for one hundred eighty (180) days from the date of the final
prospectus related to the offering. The Company may impose stop-transfer
instructions with respect to such securities subject to the foregoing
restriction until the end of said period."

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         IN WITNESS WHEREOF, the parties hereto have executed this Agreement
as of the 1st of August, 2000.

PROBUSINESS SERVICES, INC. (successor to ProBusiness, Inc.)

By  /s/ Thomas H. Sinton
  ----------------------------
Thomas H. Sinton
President and Chief Executive Officer

GENERAL ATLANTIC PARTNERS, 39, L.P.
GAP COINVESTMENT PARTNERS, L.P.
General Atlantic Service Corporation
3 Pickwick Plaza
Greenwich, Connecticut 06830

------------------------------
(Printed Name of Purchaser)

------------------------------
(Signature)

------------------------------
(Title, if Applicable)


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        IN WITNESS WHEREOF, the parties hereto have executed this Agreement as
of the 1st of August, 2000.

                                     GENERAL ATLANTIC PARTNERS 39, L.P.

                                     By:      GENERAL ATLANTIC PARTNERS, LLC,
                                                its General Partner

                                              By: /s/ Steven A. Denning
                                                 -----------------------------
                                                 Name:  Steven A. Denning
                                                 Title: A Managing Member

                                     GAP COINVESTMENT PARTNERS, L.P.

                                     By: /s/ Steven A. Denning
                                        --------------------------------------
                                        Name:  Steven A. Denning
                                        Title: A General Partner


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                  IN WITNESS WHEREOF, the parties hereto have executed this
   Agreement as of the 1st of August, 2000.

                                     GENERAL ATLANTIC PARTNERS 70, L.P.

                                     By:      GENERAL ATLANTIC PARTNERS, LLC,
                                                its General Partner

                                              By: /s/ Steven A. Denning
                                                 -----------------------------
                                              Name:  Steven A. Denning
                                              Title: A Managing Member

                                     GAP COINVESTMENT PARTNERS II, L.P.

                                     By: /s/ Steven A. Denning
                                        --------------------------------------
                                     Name:  Steven A. Denning
                                     Title: A General Partner

                                     GAPSTAR, LLC

                                     By:      GENERAL ATLANTIC PARTNERS, LLC,
                                                its Managing Member

                                              By: /s/ Steven A. Denning
                                                 -----------------------------
                                              Name:  Steven A. Denning
                                              Title: A Managing Member

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