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                                                                   Exhibit 10.27

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                         QUEST DIAGNOSTICS INCORPORATED
                                              AS ISSUER


                              THE BANK OF NEW YORK
                                              AS TRUSTEE

                     THE SUBSIDIARY GUARANTORS NAMED HEREIN
                                              AS SUBSIDIARY GUARANTORS

                        ---------------------------------

                          Fourth Supplemental Indenture

                            Dated as of July 21, 2000

                         -------------------------------




                                  $150,000,000

                   10 3/4 % Senior Subordinated Notes due 2006

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                          FOURTH SUPPLEMENTAL INDENTURE

                  Fourth Supplemental Indenture (this "Supplemental Indenture")
dated as of July 21, 2000 between QUEST DIAGNOSTICS INCORPORATED, a Delaware
corporation (the "Company") and THE BANK OF NEW YORK, as Trustee (the
"Trustee").

                                   WITNESSETH:

                  WHEREAS, in accordance with Section 902 of the Indenture
relating to the 10 3/4% Senior Subordinated Notes due 2006 of the Company (the
"Securities"), dated as of December 16, 1996 (the "Indenture"), the Trustee, the
Company and the Holders of at least a majority in principal amount at maturity
of the Securities outstanding as of the date hereof desire to amend certain
terms of the Indenture as described below;

                  WHEREAS, the Company intends to effect a receivables financing
transaction whereby the Company's wholly owned subsidiary, Quest Diagnostics
Receivables, Inc., a Delaware corporation, shall acquire accounts receivable
from the Company and some Restricted Subsidiaries from time to time;

                  WHEREAS, in accordance with the terms of the Indenture,
Holders of in excess of a majority in aggregate principal amount of the
outstanding Securities have consented to the amendments set forth herein;

                  WHEREAS, the execution and delivery of this Supplemental
Indenture has been duly authorized by the parties hereto and the Trustee has
received an Opinion of Counsel pursuant to Section 903 of the Indenture, and all
other acts necessary to make this Supplemental Indenture a valid and binding
supplement to the Indenture and effectively amending the Indenture as set forth
herein have been duly taken;

                  NOW, THEREFORE, the parties hereto agree as follows:

                  SECTION 101.  DEFINITIONS.

                  All capitalized terms used and not defined herein shall have
the meanings ascribed thereto in the Indenture.

                  SECTION 201.  AMENDMENTS TO THE INDENTURE.

                  Subject to Section 3 hereof, the Indenture is hereby amended
as follows:

                  (a) AMENDMENT TO SECTION 101. (1) Section 101 of the Indenture
is hereby amended by adding the following definitions:

                  "Quest Diagnostics Receivables" means Quest Diagnostics
Receivables Inc., the entity carrying out the transactions contemplated under
the Receivables Credit Facility and the Sale Agreement or any successor entity
thereof.


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                  "Receivables Credit Facility" means the Revolving Credit
Facility among the Company, as initial servicer, Quest Diagnostics Receivables,
Blue Ridge Asset Funding Corporation and Wachovia Bank, N.A., as amended from
time to time, which shall be entered into on or before September 30, 2000,
including any and all renewals, refinancings, refundings, replacements thereof
and renewals of such facility or any successor facility.

                  "Receivables Financing" means the receivables financing
transaction contemplated by the Receivables Credit Facility, PROVIDED, HOWEVER,
that:

                  (i)      All accounts receivable and certain related rights
                  acquired by Quest Diagnostics Receivables from the Company and
                  the Restricted Subsidiaries from time to time shall be
                  acquired on arms-length and commercially reasonable terms
                  pursuant to the Sale Agreement;

                  (ii)     The receivables shall be transferred to Quest
                  Diagnostics Receivables by one or more of the following
                  methods:

                           -   by the Company or the Restricted Subsidiaries in
                               exchange for cash collected by Quest Diagnostics
                               Receivables from receivables previously
                               transferred to it;

                           -   by the Company or the Restricted Subsidiaries in
                               exchange for cash borrowed by Quest Diagnostics
                               Receivables under the Receivables Credit
                               Facility;

                           -   by the Company or the Restricted Subsidiaries in
                               exchange for subordinated loans from the Company
                               or the Restricted Subsidiaries, as the case may
                               be, to Quest Diagnostics Receivables, each such
                               subordinated loan to be evidenced by a promissory
                               note; or

                           -   by the Company as a capital contribution of
                               receivables to Quest Diagnostics Receivables;

                  (iii)    Any loans or advances from Quest Diagnostics
                  Receivables to the Company shall be in the form of demand
                  loans or advances bearing market interest rates to be agreed
                  by the parties from time to time as permitted under the
                  Receivables Credit Facility; and

                  (iv)     Quest Diagnostics Receivables shall grant a security
                  interest in the receivables, its demand advances to the
                  Company and its rights under the Sale Agreement to the lenders
                  under the Receivables Credit Facility to secure the
                  performance of its obligations under the Receivables Credit
                  Facility.

                  "Sale Agreement" means the Receivables Sale Agreement among
Quest Diagnostics Receivables, the Company and certain Subsidiary Guarantors
relating to the sale of accounts receivable by the Company and certain
Subsidiary Guarantors to Quest Diagnostics Receivables, as amended from time to
time, including any and all renewals and replacements of such agreement or any
successor agreement.

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                  (2) Section 101 of the Indenture is hereby further amended by
replacing the definition of "Permitted Investment" in its entirety with the
following:

                  "Permitted Investment" means (i) any Investment in a Wholly
Owned Subsidiary of such Person, (ii) securities either issued directly or fully
guaranteed or insured by the government of the United States of America or any
agency or instrumentality thereof (provided that the full faith and credit of
the United States is pledged in support thereof) having maturities of not more
than thirteen months from the date of acquisition thereof by such Person; (iii)
time deposits and certificates of deposit, having maturities of not more than
one year from the date of deposit, of any bank or trust company organized under
the laws of the United States of America or any state thereof having capital and
surplus in excess of $500 million and having peer group rating of B or better
(or the equivalent thereof) by Thompson BankWatch, Inc. or outstanding long-term
debt rated BBB or better (or the equivalent thereof) by S&P or Baa or better (or
the equivalent thereof) by Moody's, (iv) repurchase obligations with a term of
not more than 30 days for underlying securities of the types described in
Clauses (ii) and (iii) above entered into with any bank meeting the
qualifications specified in Clause (iii) above, (v) commercial paper or master
notes issued or guaranteed by an entity (other than commercial paper or master
notes issued or guaranteed by an Affiliate or Related Person) rated A-1 or the
equivalent thereof by S&P or P-1 (or the equivalent thereof) by Moody's, and in
each case maturing within 180 days, (vi) securities with maturities of six
months or less from the date of acquisition issued or fully guaranteed by any
state, commonwealth or territory of the United States of America, or by any
political subdivision or taxing authority thereof and rated at least A by S&P or
MIG 2 by Moody's, (vii) money market funds rated AAm (or the equivalent thereof)
by S&P or Aa (or the equivalent thereof) by Moody's or bond funds rated AAf (or
the equivalent thereof) by S&P or Aa (or the equivalent thereof) by Moody's that
invest in the foregoing items, (viii) any Investment in a Person that, as a
consequence of such Investment, becomes a Restricted Subsidiary and that is
engaged in a Permitted Business if (A) the Company would, at the time of such
Investment and after giving pro forma effect thereto as if such Investment had
been made at the beginning of the most recently ended four full fiscal quarter
period for which internal financial statements are available immediately
preceding the date of such Investment, have been permitted to Incur at least
$1.00 of additional Debt pursuant to the Consolidated EBITDA Coverage Ratio test
set forth in the first paragraph of Section 1008 and (B) immediately after
giving effect to such Investment, the Company would have a Consolidated Net
Worth not less than 95% of the Consolidated Net Worth of the Company immediately
prior to such Investment, (ix) receivables owing to the Company or a Subsidiary
of the Company if created or acquired in the ordinary course of business and
payable or dischargeable in accordance with customary trade terms, (x)
extensions of trade credit made in the ordinary course of business and on
customary terms, (xi) the letter of credit issued pursuant to the Credit
Facility in favor of Kenneth W. Freeman to secure his pension benefits in an
amount not to exceed $10 million, (xii) any Investment by the Company or a
Restricted Subsidiary in Quest Diagnostics Receivables pursuant to the
Receivables Financing; (xiii) any Investment in addition to Investments
permitted to be made by Clauses (i) through (xii) above if the aggregate amount
(including cash and the fair value of property other than cash, as determined by
the Board of Directors) of such Investment, together with all other investments
made pursuant to this Clause (xiii) and then held by the Company and its
Restricted Subsidiaries (determined as of the time made), does not exceed $5
million.

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                  (b) AMENDMENT TO SECTION 1008. Section 1008 of the Indenture
is hereby amended by replacing clause (iv) of this section in its entirety with
the following:

                      (iv)     Debt owed by the Company to any Wholly Owned
                  Restricted Subsidiary or Quest Diagnostics Receivables
                  pursuant to the Receivables Financing or Debt owed by a Wholly
                  Owned Restricted Subsidiary to the Company; PROVIDED, HOWEVER,
                  that (a) any such Debt owing by the Company to a Wholly Owned
                  Restricted Subsidiary shall be Subordinated Debt and (b) upon
                  either (1) the transfer or other disposition by such Wholly
                  Owned Restricted Subsidiary, Quest Diagnostics Receivables or
                  the Company of any Debt so permitted to a Person other than
                  the Company or another Wholly Owned Restricted Subsidiary or
                  (2) the issuance (other than directors' qualifying shares),
                  sale, lease, transfer or other disposition of shares of
                  Capital Stock (including by consolidation or merger) of such
                  Wholly Owned Restricted Subsidiary or Quest Diagnostics
                  Receivables to a Person other than the Company or another such
                  Wholly Owned Restricted Subsidiary or, in the case of Quest
                  Diagnostics Receivables, other than a Wholly Owned Subsidiary
                  in accordance with Section 1022(d)(iv), the provisions of this
                  Clause (iv) shall no longer be applicable to such Debt and
                  such Debt shall be deemed to have been Incurred at the time of
                  such transfer or other disposition or such issuance, sale,
                  lease, transfer or other disposition;

                  (c) AMENDMENT OF SECTION 1015. Section 1015 of the Indenture
is hereby amended by inserting the phrase "or (vii) such transaction is entered
into pursuant to the Receivables Financing;" after "and (c) if the Consideration
is greater than $15 million, then the determinations referred to in Clause (a)
above, in addition to the action required by Clause (b) above, must also be
confirmed by a nationally recognized investment banking firm (which may not be
an Affiliate or Related Person of the Company), in a written opinion delivered
to the Board of Directors of the Company prior to consummation of such
transaction (or series of transactions);"

                  (d) AMENDMENT OF SECTION 1019. Section 1019 of the Indenture
is hereby amended by:

                  a.           inserting the phrase "PROVIDED, HOWEVER, that
                               notwithstanding the foregoing, Quest Diagnostics
                               Receivables may be designated as an Unrestricted
                               Subsidiary on the date of the Supplemental
                               Indenture; PROVIDED, FURTHER, that if Quest
                               Diagnostics Receivables is so designated, it
                               shall remain an Unrestricted Subsidiary for as
                               long as the Receivables Credit Facility remains
                               outstanding." after "and (iii) such change would
                               not otherwise result (after the giving of notice
                               or the lapse of time, or both) in an Event of
                               Default" and

                  b.           inserting the phrase "provided that the
                               Receivables Credit Facility shall not be
                               Unpermitted Debt" at the end of the section after

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                               "(y) such Debt is secured by a Lien on any
                               property or other assets of the Company and any
                               of its Restricted Subsidiaries."

                  (e) ADDITION OF NEW SECTION. The following Section 1022 shall
be inserted:

                  SECTION 1022.  RESTRICTIONS RELATING TO RECEIVABLES FINANCING.

                  (a)          The Company shall cause Quest Diagnostics
                               Receivables to pay a dividend to the Company on
                               the date of the Supplemental Indenture in an
                               amount equal to the initial amount borrowed by
                               Quest Diagnostics Receivables under the
                               Receivables Credit Facility on the date of the
                               Receivables Credit Facility (such amount is
                               estimated to be approximately $256 million). The
                               Company shall use all of the dividend it receives
                               from Quest Diagnostics Receivables on the date of
                               the Supplemental Indenture to permanently pay
                               down Debt under the term loan portion (but not
                               the revolving portion) of the Credit Agreement
                               dated as of August 16, 1999, as amended from time
                               to time, among the Company; the guarantors party
                               thereto; the banks named therein; Merrill Lynch &
                               Co.; Merrill Lynch, Pierce, Fenner & Smith
                               Incorporated, as joint lead arranger and
                               syndication agent; Banc of America Securities
                               LLC, as joint lead arranger; Bank of America,
                               N.A., as administrative agent; Wachovia Bank,
                               N.A., as co-documentation agent and The Bank of
                               New York, as co-documentation agent, which term
                               loan was Incurred pursuant to the first paragraph
                               of Section 1008.

                  (b)          The Company shall permanently reduce the amount
                               of term loans borrowed under the credit agreement
                               described in subsection (a) or any and all
                               renewals of, refinancings, refundings and
                               replacements of such facility or any successor
                               facility by an amount equal to the excess of (x)
                               the total amount borrowed by Quest Diagnostics
                               Receivables under the Receivables Credit Facility
                               over (y) the initial amount borrowed by Quest
                               Diagnostics Receivables under the Receivables
                               Credit Facility on the date of the Receivables
                               Credit Facility.

                  (c)          The Company shall not sell, transfer or otherwise
                               dispose of its shares in Quest Diagnostics
                               Receivables to any Person other than to a
                               Restricted Subsidiary and except as permitted
                               under paragraph (d)(iv) below.

                  (d)          The Company shall cause Quest Diagnostics
                               Receivables to comply with the following
                               covenants:

                               (i)  Quest Diagnostics Receivables will be a
                                    limited purpose entity whose primary
                                    activities are

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                                    restricted to purchasing or otherwise
                                    acquiring accounts receivable from the
                                    Company and any of its Subsidiaries,
                                    entering into agreements for the financing
                                    and servicing of the account receivables,
                                    and conducting such other activities as it
                                    deems necessary or appropriate to carry out
                                    its primary activities contemplated by the
                                    Receivables Credit Facility and the Sale
                                    Agreement.

                               (ii) Quest Diagnostics Receivables will not incur
                                    or permit to exist any Debt except: (x) Debt
                                    under the Receivables Credit Facility and
                                    (y) Debt owing to the Company or any Wholly
                                    Owned Subsidiary of the Company.

                               (iii) Quests Diagnostics Receivables will not
                                    Incur any Liens other than the Liens under
                                    the Receivables Credit Facility and Liens
                                    permitted under clause (ix) of the
                                    definition of Permitted Liens.

                               (iv) Quest Diagnostics Receivables will not merge
                                    into or consolidate with any other Person,
                                    or permit any other Person to merge into or
                                    consolidate with it, or purchase, lease or
                                    otherwise acquire (in one transaction or a
                                    series of transactions) assets that would be
                                    material to Quest Diagnostics Receivables
                                    (whether directly by purchase, lease or
                                    other acquisition of the assets of such
                                    Person or indirectly by purchase or other
                                    acquisition of the capital stock of such
                                    other Person) other than (x) the acquisition
                                    of the receivables and other assets pursuant
                                    to the Sale Agreement and (y) any
                                    transaction, including a merger or
                                    consolidation with a Wholly Owned Subsidiary
                                    of the Company that has no Debt outstanding
                                    other than Debt owing to the Company or
                                    another Wholly Owned Subsidiary of the
                                    Company and whose activities are restricted
                                    to the same extent as Quest Diagnostics
                                    Receivables as set forth in this Section
                                    1022(d), provided that the Company shall be
                                    bound by the same restriction as set forth
                                    in Section 1022(c) with respect to such
                                    Wholly Owned Subsidiary.

                  SECTION 301. CONFIRMATION; EFFECTIVENESS. As amended by this
Supplemental Indenture, the Indenture and the Securities are ratified and
confirmed in all respects and the Indenture as so amended shall be read, taken
and construed as one and the same instrument. The provisions of this
Supplemental Indenture shall become operative as of the date of this

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Supplemental Indenture. This Supplemental Indenture may be executed in any
number of counterparts, each of which counterparts together shall constitute but
one and the same instrument.

                  SECTION 401. TRUST INDENTURE ACT. If and to the extent that
any provision of this Supplemental Indenture limits, qualifies or conflicts with
another provision included in this Supplemental Indenture or in the Indenture,
which is required to be included in this Supplemental Indenture or the Indenture
by the Trust Indenture Act of 1939, as amended (the "TIA"), such required
provision of the TIA shall control.

                  SECTION 501. GOVERNING LAW. This Supplemental Indenture shall
be deemed governed by, and construed in accordance with, the internal laws of
the State of New York, but without giving effect to applicable principles of
conflicts of law thereof to the extent that the application of the laws of
another jurisdiction would be required thereby.

                  SECTION 601. RIGHTS OF TRUSTEE. Without limiting any other
protections or rights afforded the Trustee at law, by contract or otherwise, the
Trustee will be entitled to the full benefits afforded by Sections 6.03 and 6.04
of the Indenture in connection with its execution and delivery of this
Supplemental Indenture.

                  IN WITNESS WHEREOF, the parties hereto have caused this
Supplemental Indenture to be duly executed as of the date first written above.

                                       QUEST DIAGNOSTICS INCORPORATED


                                       By   /s/ Kenneth R. Finnegan
                                         ------------------------------
                                       Title:   Vice President

                                       THE BANK OF NEW YORK, as Trustee

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

                                       QUEST DIAGNOSTICS
                                       INVESTMENTS INCORPORATED

                                       QUEST DIAGNOSTICS
                                       FINANCE INCORPORATED

                                       By  /s/ Peter C. Fulweiler
                                         ------------------------------
                                       Title:  President

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                                       QUEST DIAGNOSTICS INCORPORATED (CA)
                                       QUEST DIAGNOSTICS INCORPORATED (CT)
                                       QUEST DIAGNOSTICS INCORPORATED (OH)
                                       QUEST DIAGNOSTICS INCORPORATED (MA)
                                       QUEST DIAGNOSTICS INCORPORATED (MD)
                                       QUEST DIAGNOSTICS INCORPORATED (MI)
                                       QUEST DIAGNOSTICS LLC (IL)
                                       QUEST DIAGNOSTICS  OF PENNSYLVANIA INC.
                                       QUEST MRL, INC.
                                       LABORATORY HOLDINGS INCORPORATED
                                       DIAGNOSTIC REFERENCE SERVICES, INC.
                                       DPD HOLDINGS, INC.
                                       METWEST INC.
                                       NICHOLS INSTITUTE DIAGNOSTICS
                                       QUEST HOLDINGS INCORPORATED (DE)
                                       QUEST HOLDINGS INCORPORATED (MD)
                                       QUEST HOLDINGS INCORPORATED (MI)
                                       QUEST DIAGNOSTICS HOLDINGS
                                          INCORPORATED (DE)
                                       QUEST DIAGNOSTICS CLINICAL
                                          LABORATORIES, INC. (DE)

                                       By   /s/ Kenneth R. Finnegan
                                         --------------------------
                                       Title:   Vice President


                                       PATHOLOGY BUILDING PARTNERSHIP

                                       By Quest Diagnostics Incorporated (MD)
                                          as General Partner

                                       By:  /s/ Kenneth R. Finnegan
                                          -------------------------
                                       Title:  Vice President







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