<Page>
Exhibit 10.1
FOURTH SUPPLEMENTAL INDENTURE
SUPPLEMENTAL INDENTURE dated as of March 19, 2003, (this
"Fourth Supplemental Indenture"), among UNILAB CORPORATION (f/k/a Quest
Diagnostics Newco Incorporated), (the "Additional Subsidiary Guarantor"), QUEST
DIAGNOSTICS INCORPORATED, a Delaware corporation (the "Company"), THE BANK OF
NEW YORK, a New York banking corporation as Trustee (the "Trustee"), and the
Subsidiary Guarantors (as defined in the Indenture referred to herein).
RECITALS OF THE COMPANY
WHEREAS, the Company, the Trustee and the Initial Subsidiary
Guarantors executed and delivered an Indenture, dated as of June 27, 2001 (the
"Base Indenture"), as supplemented by the first Supplemental Indenture as of
June 27, 2001 (the "First Supplemental Indenture"), as further supplemented by
the second Supplemental Indenture as of November 26, 2001 (the "Second
Supplemental Indenture") as further supplemented by the third Supplemental
Indenture as of April 4, 2002 (the "Third Supplemental Indenture"), and as
further supplemented by this Fourth Supplemental Indenture (collectively, the
"Indenture"), to provide for the issuance by the Company from time to time of
Securities to be issued in one or mores series as provided in the Indenture;
WHEREAS, Additional Subsidiary Guarantor intends to guarantee
the Securities under the Indenture and the issuance of guarantees has been
authorized by resolutions adopted by the Board of Directors of Additional
Subsidiary Guarantor;
WHEREAS, Section 901(11) of the Indenture provides that
without the consent of Holders of the Securities of any series issued under the
Indenture, the Company, when authorized by a Board Resolution, and the Trustee
may enter into one or more indentures supplemental to the Indenture to add a
guarantor or guarantors for any series or all series of the Securities;
WHEREAS, pursuant to Sections 904, 1601, 1602 and 1604 of the
Indenture, by delivery of a Supplemental Indenture to the Trustee in accordance
with the terms of the Indenture, each Person that becomes a Subsidiary Guarantor
after the date of the Base Indenture will be deemed to have executed and
delivered the Subsidiary Guarantee for the benefit of the Holder of the Security
upon which the Subsidiary Guarantee is endorsed, with the same effect as if such
Subsidiary Guarantor had been named thereon and had executed and delivered the
Subsidiary Guarantee;
WHEREAS, all things necessary to make this Fourth Supplemental
Indenture a valid supplement to the Indenture according to its terms and the
terms of the Indenture have been done.
<Page>
NOW, THEREFORE, for and in consideration of the foregoing, the
parties hereto hereby enter into this Fourth Supplemental Indenture, for the
equal and proportionate benefit of all Holders of the Securities, as follows:
SECTION 1 Certain Terms Defined in the Indenture.
All capitalized terms used but not defined herein shall have
the meanings assigned to them in the Indenture.
SECTION 2 Agreement to Guarantee.
Additional Subsidiary Guarantor, by its signature below,
agrees to become an Additional Subsidiary Guarantor under the Indenture with the
same force and effect as if originally named therein as a Subsidiary Guarantor.
Additional Subsidiary Guarantor hereby agrees to all the terms and provisions of
the Indenture applicable to it as Additional Subsidiary Guarantor thereunder and
each reference to a "Subsidiary Guarantor" in the Indenture shall be deemed to
include Additional Subsidiary Guarantor.
SECTION 3 Indenture remains in Full Force and Effect.
Except as expressly supplemented by this Fourth Supplemental
Indenture, the Indenture shall remain in full force and effect in accordance
with its terms.
SECTION 4 New York Law to Govern.
THIS FOURTH SUPPLEMENTAL INDENTURE SHALL BE GOVERNED BY AND
CONSTRUED IN ACCORDANCE WITH THE LAWS OF THE STATE OF NEW YORK, WITHOUT REGARD
TO PRINCIPLES OF CONFLICT OF LAWS.
SECTION 5 Separability.
In case any one or more of the provisions contained in this
Fourth Supplemental Indenture should be held invalid, illegal or unenforceable
in any respect, the validity, legality and enforceability of the remaining
provisions contained herein and in the Indenture shall not in any way be
affected or impaired. The parties hereto shall endeavor in good faith
negotiations to replace any invalid, illegal or enforceable provisions herein
with valid provisions, the economic effect of which comes as close as possible
to that of the invalid, illegal or unenforceable provisions.
SECTION 6 May be Executed in Counterparts.
This Fourth Supplemental Indenture may be executed in
counterparts, each of which when taken together shall constitute one and the
same instrument. This Fourth Supplemental Indenture shall become effective when
the Trustee shall have received a counterpart thereof that bears the signature
of the Additional Subsidiary Guarantor.
2
<Page>
IN WITNESS WHEREOF, the Company, the Additional Subsidiary
Guarantor, and the Trustee have duly executed this Fourth Supplemental Indenture
as of the day and year first above written.
Attest: QUEST DIAGNOSTICS INCORPORATED,
a Delaware corporation, as Company
___________________ By:
----------------------------------------
Leo C. Farrenkopf, Jr. Name: Joseph P. Manory
Secretary Title: Vice President and Treasurer
Attest: UNILAB CORPORATION,
as Additional Subsidiary Guarantor
- --------------------
Leo C. Farrenkopf, Jr. By:_____________________________________
Secretary Name: Joseph P. Manory
Title: Vice President and Treasurer
Attest: THE BANK OF NEW YORK, as Trustee
_______________________ By:
-------------------------------------
Title:__________________________________