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                                                                   EXHIBIT 10.21

                                FOURTH AMENDMENT

     THIS FOURTH AMENDMENT (this "AMENDMENT") dated as of December 23, 2002 to
the Credit Agreement referenced below is by and among Medical Staffing Network,
Inc., a Delaware corporation (the "BORROWER"), Medical Staffing Holdings, LLC, a
Delaware limited liability company (the "PARENT"), the Subsidiaries of the
Borrower identified as "Guarantors" on the signature pages hereto (together with
the Parent, the "GUARANTORS"), the Lenders identified on the signature pages
hereto and Bank of America, N.A., as Administrative Agent.

                               W I T N E S S E T H

     WHEREAS, a $120 million credit facility has been extended to the Borrower
pursuant to the terms of that Credit Agreement (as amended, modified and
supplemented from time to time, the "CREDIT AGREEMENT") dated as of October 26,
2001 among the Borrower, the Guarantors, the Lenders, LaSalle Bank, National
Association, as syndication agent, and General Electric Capital Corporation,
Barclays Bank, PLC, and Antares Capital Corporation, as co-documentation agents,
and Bank of America, N.A., as Administrative Agent;

     WHEREAS, the Borrower has requested certain modifications to the Credit
Agreement;

     WHEREAS, the requested modifications require the approval of all the
Lenders; and

     WHEREAS, all the Lenders have agreed to the requested modifications on the
terms and conditions set forth herein.

     NOW, THEREFORE, IN CONSIDERATION of the premises and other good and
valuable consideration, the receipt and sufficiency of which are hereby
acknowledged, the parties hereto agree as follows:

     1.   CAPITALIZED TERMS. Capitalized terms used herein but not otherwise
defined herein shall have the meanings provided to such terms in the Credit
Agreement.

     2.   AMENDMENTS. Section 8.13 of the Credit Agreement is amended to read as
follows:

          8.13   CAPITAL EXPENDITURES.

                 THE CREDIT PARTIES WILL NOT PERMIT AGGREGATE CAPITAL
                 EXPENDITURES FOR (A) THE FISCAL QUARTER ENDING ON OR ABOUT
                 DECEMBER 31, 2001 TO EXCEED $1,500,000 AND (B) EACH FISCAL YEAR
                 SET FORTH BELOW TO EXCEED:
<Table>
                 <S>                                                       <C>
                 Fiscal year ending on or about December 31, 2002          $  9,000,000
                 Fiscal year ending on or about December 31, 2003          $  5,750,000
                 Fiscal year ending on or about December 31, 2004          $  7,300,000
                 Fiscal year ending on or about December 31, 2005          $  8,300,000
                 Fiscal year ending on or about December 31, 2006          $ 10,300,000
</Table>

     3.   CONDITIONS PRECEDENT. This Amendment shall become effective
immediately upon receipt by the Administrative Agent of counterparts of this
Amendment duly executed by the Credit Parties and the Required Lenders.

     4.   REAFFIRMATION OF REPRESENTATIONS AND WARRANTIES. The Borrower affirms
that the representations and warranties set forth in the Credit Agreement and
the other Credit Documents (as hereby amended) are true and correct as of the
date hereof (except those that expressly relate to an earlier period).

     5.   REAFFIRMATION OF GUARANTY. Each Guarantor (i) acknowledges and
consents to all of the terms and conditions of this Amendment, (ii) affirms all
of its obligations under the Credit Documents and (iii) agrees that this
Amendment and all documents executed in connection herewith do not operate to
reduce or discharge such Guarantor's obligations under the Credit Agreement or
the other Credit Documents.

     6.   REAFFIRMATION OF SECURITY INTERESTS. The Borrower and each Guarantor
(i) affirms that each of the Liens granted in or pursuant to the Credit
Documents are valid and subsisting and (ii) agrees that this Amendment shall in
no manner impair or otherwise adversely effect any of the Liens granted in or
pursuant to the Credit Documents.

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     7.   NO OTHER CHANGES. Except as modified hereby, all of the terms and
provisions of the Credit Agreement and the other Credit Documents (including
schedules and exhibits thereto) shall remain in full force and effect.

     8.   COUNTERPARTS. This Amendment may be executed in any number of
counterparts, each of which when so executed and delivered shall be deemed an
original and it shall not be necessary in making proof of this Amendment to
produce or account for more than one such counterpart. Delivery of an executed
counterpart of this Amendment by telecopy by any party hereto shall be effective
as such party's original executed counterpart and shall constitute a
representation that such party's original executed counterpart will be
delivered.

     9.   NO NOVATION. The execution and delivery of this Amendment shall not
constitute a novation of any indebtedness or other obligations owing to the
Lenders under the Credit Agreement based on any facts or events occurring or
existing prior to the execution and delivery of this Amendment. On the date
hereof, the credit facilities described in the Credit Agreement shall be amended
and supplemented as described in this Amendment, and all loans and other
obligations of the Borrower and Guarantors outstanding as of the date hereof
under the Credit Agreement shall be deemed to be loans and obligations
outstanding under the Credit Agreement as amended, without further action by any
Person.

     10.  GOVERNING LAW. This Amendment shall be deemed to be a contract made
under, and for all purposes shall be construed in accordance with, the laws of
the State of New York.

                            [Signature Pages Follow]

                                        2
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     IN WITNESS WHEREOF, each of the parties hereto has caused a counterpart of
this Fourth Amendment to be duly executed and delivered as of the date first
above written.

BORROWER:            MEDICAL STAFFING NETWORK, INC.,
                     a Delaware corporation

                     By: /s/ Kevin S. Little
                        -----------------------------------------
                     Name:  Kevin S. Little
                     Title: Chief Financial Officer

GUARANTORS:          MEDICAL STAFFING HOLDINGS, LLC,
                     a Delaware limited liability company
                     By:   Medical Staffing Network Holdings, Inc.
                           (formerly known as MSN Holdings, Inc.),
                           a Delaware corporation, its sole member

                           By: /s/ Kevin S. Little
                              -----------------------------------------
                           Name:  Kevin S. Little
                           Title: Chief Financial Officer

                           [Signature Pages Continue]

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AGENT:           BANK OF AMERICA, N.A., in its capacity as Administrative Agent

                 By: /s/ James W. Ford
                    -----------------------------------------
                 Name:  James W. Ford
                 Title: Managing Director

LENDERS:         BANK OF AMERICA, N.A., in its capacity as a Lender

                 By: /s/ James W. Ford
                    -----------------------------------------
                 Name:  James W. Ford
                 Title: Managing Director

                 LASALLE BANK, NATIONAL ASSOCIATION

                 By: /s/ Pat Laughlin
                    -----------------------------------------
                 Name:  Pat Laughlin
                 Title:

                 GENERAL ELECTRIC CAPITAL CORPORATION

                 By:
                    -----------------------------------------
                 Name:
                 Title:

                 ANTARES CAPITAL CORPORATION

                 By: /s/ John G. Martin
                    -----------------------------------------
                 Name:  John G. Martin
                 Title: Managing Director

                 JP MORGAN CHASE BANK, as trustee of the
                 ANTARES FUNDING TRUST created under the
                 Trust Agreement dated as of November 30, 1999

                 By: /s/ Shelly Sterling
                    -----------------------------------------
                 Name:  Shelly Sterling
                 Title: Assistant Vice President

                 JPMORGAN CHASE BANK

                 By:
                    -----------------------------------------
                 Name:
                 Title:

                           [Signature Pages Continue]

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                 VAN KAMPEN PRIME RATE INCOME TRUST

                 By:   Van Kampen Investment Advisory Corp.

                       By: /s/ Brad Langs
                          -----------------------------------------
                       Name:  Brad Langs
                       Title: Vice President

                 VAN KAMPEN SENIOR INCOME TRUST

                 By:   Van Kampen Investment Advisory Corp.

                       By: /s/ Brad Langs
                          -----------------------------------------
                       Name:  Brad Langs
                       Title: Vice President

                 VAN KAMPEN SENIOR FLOATING RATE FUND

                 By:   Van Kampen Investment Advisory Corp.

                       By:
                          -----------------------------------------
                       Name:
                       Title: