Exhibit 10.1

CONFIDENTIAL

Amendment # 4 to the
Letter of Agreement between
SmithKline Beecham Corporation and
Quest Diagnostics Incorporated

1.      The purpose of this Amendment # 4 to the Letter of Agreement effective
January 1, 2008 between SmithKline Beecham Corporation d/b/a GlaxoSmithKline and
Quest Diagnostics Incorporated (hereinafter the “LOA”) is to extend the term
referenced in section 2 of the LOA to the earlier of (i) September 30, 2008, or
(ii) the full execution of a new comprehensive Globale Clinical Trials
Agreement.   2.      All other terms and conditions of the LOA shall remain in
full force and effect.   3.      The parties hereto agree to this Amendment #4
to the LOA by their authorized signatures below.

SMITHKLINE BEECHAM CORPORATION   QUEST DIAGNOSTICS INCORPORATED       /s/ Paula
M. Russella   /s/ Daniel P. Megronigle Paula M. Russella
Sourcing Group Manager   Daniel P. Megronigle
Executive Director, Sales & Marketing       25 July 2008   21 July 2008 Date  
Date      

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