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                      FIFTH AMENDMENT AND FOURTH WAIVER TO
                                        
                                        
                                CREDIT AGREEMENT
                                        
                                        
                          Dated as of December 23, 1996
                                        
                                        
                                      Among
                                        
                                        
                   LABORATORY CORPORATION OF AMERICA HOLDINGS,
                                  as Borrower,
                                  -----------
                                        
                                        
                             THE BANKS NAMED HEREIN,
                                  as Banks, and
                                  --------
                                        
                                        
                        CREDIT SUISSE (NEW YORK BRANCH),
                             as Administrative Agent
                             -----------------------           
                                        
                                        
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              FIFTH AMENDMENT AND FOURTH WAIVER TO CREDIT AGREEMENT
                                        
                                        
                                        
          FIFTH  AMENDMENT AND FOURTH WAIVER TO CREDIT AGREEMENT,  dated  as  of
December  23,  1996 (this "Amendment") among LABORATORY CORPORATION  OF  AMERICA
HOLDINGS  (formerly  known  as NATIONAL HEALTH LABORATORIES  HOLDINGS  INC.),  a
Delaware  corporation  (the "Borrower"), the banks, financial  institutions  and
other  institutional lenders (the "Banks") listed on the signature pages hereof,
and  CREDIT  SUISSE  (NEW  YORK  BRANCH) ("CS"), as  administrative  agent  (the
"Administrative Agent") for the Lenders hereunder.


                             PRELIMINARY STATEMENTS
                                        
          The  parties hereto (i) have entered into a Credit Agreement dated  as
of  April  28,  1995 (as amended, the "Credit Agreement") providing  for,  among
other  things, the Lenders to lend to the Borrower up to $1,250,000,000  on  the
terms  and subject to the conditions set forth therein and (ii) desire to  amend
the Credit Agreement in the manner set forth herein.  Each capitalized term used
but  not  defined herein shall have the meaning ascribed thereto in  the  Credit
Agreement.

          NOW,  THEREFORE,  in  consideration of the  premises  and  the  mutual
covenants  and agreements contained herein, the parties hereto hereby  agree  as
follows:


                                    ARTICLE I
                                        
                                   AMENDMENTS
                                        
          SECTION  1.01.  Amendment of Definitions.  Section 1.01 of the  Credit
Agreement is hereby amended as follows:

          (a) by adding alphabetically the following new definition:

          "'Roche Debt' means the unsecured Debt of the Borrower issued in favor
of  Roche  in an aggregate amount not to exceed $187 million, such Debt  to  (i)
rank  pari passu in right of payment with the Obligations of the Borrower  under
the  Loan  Documents,  (ii)  bear interest at a rate  per  annum  equal  to  the
Eurodollar Rate plus 1.0% and (iii) be due and payable on March 31, 1997."

          SECTION  1.02.  Amendment of Negative Covenants.  Section  5.02(j)  of
the Credit Agreement is hereby amended as follows:

          (a) by deleting the word "and" at the end of Section 5.02(j)(viii) and
by  deleting  the period at the end of Section 5.02(j)(ix) and by  inserting  in
lieu thereof ";and".

          (b)  by inserting the following new subsection 5.02(j)(x), to read  in
its entirety as follows:

          "(x)   the  Roche  Debt; provided that at least $180  million  of  the
proceeds  thereof is applied to pay amounts due pursuant to the settlement  with
the  Office  of  Inspector General of the U.S. Department of  Health  and  Human
Services."

                                   ARTICLE II
                                        
                                     WAIVERS
                                        
          SECTION 2.01.  Extension of Third Waiver.  The undersigned Required
Lenders hereby agree that the Third Waiver to Credit Agreement dated as of
November 4, 1996, by the Required Lenders, in favor of the Borrower (the
"Waiver"), shall remain in effect through January 31, 1997 notwithstanding the
settlement with the Office of Inspector General of the U.S. Department of 
Health and Human Services referred to in Section 1.02 of the Waiver.

          SECTION 2.02.  Roche Debt.  The undersigned Required Lenders hereby
agree as follows:

          (a) the Roche Debt shall be excluded from the calculation of the
Borrower's Consolidated Debt for the Borrower's four fiscal quarters ending
December 31, 1996 and March 31, 1997 for the purpose of determining the
Borrower's compliance with the covenant set forth in Section 5.01(i) of the
Credit Agreement [Leverage Ratio].

          (b) the Roche Debt shall be excluded from the calculation of the
Interest Coverage Ratio for the Borrower's four fiscal quarters ending December
31, 1996 and March 31, 1997 for the purpose of determining compliance with the
covenant set forth in Section 5.01(j) of the Credit Agreement [Interest 
Coverage Ratio].

                                   ARTICLE III
                                        
                         REPRESENTATIONS AND WARRANTIES
                                        
          SECTION  3.01.   Representations and Warranties of the Borrower.   The
Borrower represents and warrants as follows:

          (a)   The  Borrower is a corporation duly organized, validly  existing
     and in good standing under the laws of the State of Delaware.
     
          (b)   The execution, delivery and performance by the Borrower of  this
     Amendment are within its corporate powers, have been duly authorized by all
     necessary corporate action, and do not contravene the Borrower's charter or
     by-laws.
     
          (c)  No authorization or approval or other action by, and no notice to
     or  filing with, any governmental authority or regulatory body is  required
     for  the  due execution, delivery and performance by the Borrower  of  this
     Amendment.
     
          (d)   This  Amendment  has been duly executed  and  delivered  by  the
     Borrower.  This Amendment is the legal, valid and binding obligation of the
     Borrower,  enforceable against the Borrower, in accordance with its  terms,
     subject to applicable bankruptcy, insolvency, reorganization, moratorium or
     similar  laws  affecting the enforceability of creditors' rights  generally
     and by general principles of equity.
     
          (e)   The representations and warranties contained in Section 4.01  of
     the  Credit Agreement are correct in all material respects on and as of the
     date hereof, as though made on and as of the date hereof.
     
          (f)   No  event  has  occurred and is continuing which  constitutes  a
     Default.
     
                                   ARTICLE IV
                                        
                                  MISCELLANEOUS
                                        
          SECTION  4.01.  Governing Law.  This Amendment shall be  governed  by,
and  construed  in accordance with, the laws of the State of New  York,  without
regard to the conflicts of law principles thereof.

          SECTION  4.02.   Execution in Counterparts.   This  Amendment  may  be
executed  in  any number of counterparts and by any combination of  the  parties
hereto in separate counterparts, each of which counterparts shall be an original
and  all  of  which taken together shall constitute one and the same instrument.
Delivery  of  an executed counterpart of a signature page to this  Amendment  by
facsimile  shall be effective as delivery of a manually executed counterpart  of
this Amendment.

          SECTION  4.03.   Effect on the Credit Agreement.  Upon  execution  and
and delivery of this Amendment and Waiver, each reference in the Credit
Agreement to "this Agreement", "hereunder", "hereof", "herein", or words of like
import shall mean  and  be  a reference to the Credit Agreement, as amended
hereby  and  each reference to the Credit Agreement in any Loan Document (as
defined in the Credit Agreement)  shall  mean and be a reference to the Credit
Agreement,  as  amended hereby.  Except as expressly modified hereby, all of the
terms and conditions of the  Credit Agreement shall remain unaltered and in full
force and effect.  This Amendment  and Waiver shall become effective as of the
date first above  written when counterparts hereof shall have been executed by
the Required Lenders.  This Amendment and Waiver is subject to the provisions of
Section 8.01 of the  Credit Agreement.

          Each of the undersigned has caused this Amendment to be executed by
its respective officer or officers thereunto duly authorized, as of the date
first written above.

BORROWER:             LABORATORY CORPORATION OF AMERICA
                        HOLDINGS


                     By:/s/ WESLEY R. ELINGBURG
                        -------------------------
                        Name: Wesley R. Elingburg
                        Title:Executive Vice President
                        and Chief Financial Officer
                     
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ADMINISTRATIVE
  AGENT:              CREDIT SUISSE (NEW YORK BRANCH),
                        as Administrative Agent


                     By:/s/ RICHARD CAREY
                        ---------------------------------
                        Name: Richard Carey
                        Title:Member of Senior Management
                     
                     and
                     
                     
                     By:/s/ KARL M. STUDER
                        ---------------------------------
                        Name: Karl M. Studer
                        Title:Member of Senior Management
                     
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                      LENDERS:              CREDIT SUISSE (NEW YORK BRANCH)
                      
                      
                      By:/s/ KARL M. STUDER
                         ----------------------------------
                         Name:  Karl M. Studer
                         Title: Member of Senior Management
                      
                      
                      By:/s/ DANIELA E. HESS
                         ----------------------
                         Name:  Daniela E. Hess
                         Title: Associate
                      
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                      BANK OF AMERICA ILLINOIS
                      
                      
                      By:
                         Name: 
                         Title: 
                                
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                      BANQUE NATIONALE DE PARIS
                      
                      
                      By:/s/ RICHARD L. STED
                         ----------------------------
                         Name:  Richard L. Sted
                         Title: Senior Vice President
                      
                      
                      By:/s/ BONNIE G. EISENSTAT
                         --------------------------
                         Name:  Bonnie G. Eisenstat
                         Title: Vice President
                      
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                      BAYERISCHE LANDESBANK GIROZENTRALE
                      
                      
                      By:/s/ WILFRIED FREUDENBERGER
                         -------------------------------
                         Name:  Wilfried Freudenberger
                         Title: Executive Vice President
                                and General Manager
               
                      
                      By:/s/ PETER OBERMANN
                         -------------------------------
                         Name:  Peter Obermann
                         Title: Senior Vice President
                                Manager Lending Division
                      
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                      THE CHASE MANHATTAN BANK
                      
                      
                      By:/s/ SCOTT S. WARD
                         ---------------------
                         Name:  Scott S. Ward
                         Title: Vice President
                      
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                      CREDIT LYONNAIS CAYMAN ISLAND BRANCH
                      
                      
                      By:/s/ PASCAL POUPELLE
                         ---------------------------
                         Name:  Pascal Poupelle
                         Title: Authorized Signature
                      
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                      DEUTSCHE BANK AG NEW YORK BRANCH
                        and/or CAYMAN ISLANDS BRANCH
                      
                      
                      By:/s/ WOLF A. KLUGE
                         ----------------------
                         Name:  Wolf A. Kluge
                         Title: Vice President
                      
                      
                      By:/s/ SHERINE FANOUS
                         -------------------------------
                         Name:  Sherine Fanous
                         Title: Assistant Vice President
                      
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                      FIRST UNION NATIONAL BANK
                      
                      
                      By:/s/ JOSEPH H. TOWELL
                         ----------------------------
                         Name:  Joseph H. Towell
                         Title: Senior Vice President
                      
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                      THE FUJI BANK, LTD. (NEW YORK BRANCH)
                      
                      
                      By:/s/ MASANOBU KOBAYASHI
                         ------------------------------
                         Name:  Masanobu Kobayashi
                         Title: Vice President & Manager
                      
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                      NATIONSBANK, N.A.
                      
                      
                      By:
                         Name:
                         Title:
                      
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                      SOCIETE GENERALE
                      
                      
                      By:/s/ R. CUENE-GRANDIDIER
                         --------------------------
                         Name:  R. Cuene-Grandidier
                         Title: Vice President
                      

                      By:/s/ GEORG L. PETERS
                         -----------------------
                         Name:  Georg L. Peters
                         Title: Vice President

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                      SUMITOMO BANK
                      
                      
                      By:/s/ SURESH S. TATA
                         ----------------------------
                         Name:  Suresh S. Tata
                         Title: Senior Vice President
                      
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                      SWISS BANK CORPORATION
                      
                      
                      By:/s/ MICHAEL A. DRISCOLL
                         ----------------------------------
                         Name:  Michael A. Driscoll
                         Title: Director Corporate Clients
                                Switzerland
                      
                      
                      By:/s/ JORG RAUTHE
                         --------------------------
                         Name:  Jorg Rauthe 
                         Title: Associate Director
                                Corporate Clients
                                Switzerland
                      
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                      WACHOVIA BANK OF GEORGIA, N.A.
                      
                      
                      By:/s/ JAMES C. RATCLIFF JR.
                         ----------------------------
                         Name:  James C. Ratcliff Jr.
                         Title: Vice President
                      
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                      WESTDEUTSCHE LANDESBANK
                      
                      
                      By:/s/ ALAN S. BOOKSPAN
                         -----------------------
                         Name:  Alan S. Bookspan
                         Title: Vice President
                      
                      
                      By:/s/ CATHERINE ROHLAND
                         ------------------------
                         Name:  Catherine Rohland
                         Title: Vice President
                      
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                      COMMERZBANK AKTIENGESELLSCHAFT,
                        Atlanta Agency
                      
                      
                      By:/s/ H. YERGEY
                         ----------------------
                         Name:  H. Yergey
                         Title: Vice President

                      By:/s/ E. KAGERER
                         ----------------------
                         Name:  E. Kagerer
                         Title: Vice President
                      
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                      BANK BRUSSELS LAMBERT,
                        New York Branch
                      
                      
                      By:/s/ JURGEN RIGTERINK
                         ------------------------
                         Name:  Jurgen Rigterink
                         Title: Vice President
  

                      By:/s/ DOMINICK H.J. VANGAEVER
                         -------------------------------
                         Name:  Dominick H.J. Vangaever
                         Title: Senior Vice President
                                Credit
                      
                      
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