Document:

Exhibit 4.1

                          THIRD SUPPLEMENTAL INDENTURE

                  SUPPLEMENTAL INDENTURE dated as of April 4, 2002, (this "Third
Supplemental Indenture"), among AMERICAN MEDICAL LABORATORIES, INCORPORATED, a
Delaware corporation, AML INC., a Delaware corporation, MEDICAL LABORATORIES
CORPORATION, a Virginia corporation, APL HEALTHCARE GROUP, INC., a Nevada
corporation, APL PROPERTIES LIMITED COMPANY, a Nevada limited company, QUEST
DIAGNOSTICS LLC (CT) a Connecticut limited liability company, QUEST DIAGNOSTICS
LLC (MA), a Massachusetts limited liability company (collectively, the
"Additional Subsidiary Guarantors"), 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") and as further supplemented by this Third 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, the Additional Subsidiary Guarantors intend to
guarantee the Securities under the Indenture and the issuance of guarantees has
been authorized by resolutions adopted by the Board of Directors of such
Additional Subsidiary Guarantors;

                  WHEREAS, Sections 901(1) and 901(11) of the Indenture provide
that without the consent of Holders of the Securities of any series issued under
the Indenture, the Company and the Subsidiary Guarantors, when authorized by a
Board Resolution, and the Trustee may enter into one or more indentures
supplemental to the Indenture to (a) evidence the successor of another Person to
any Subsidiary Guarantor and the assumption by such successor of the covenants
of such Subsidiary Guarantor in the Indenture and in the Securities and (b) 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;

<PAGE>

                  WHEREAS, all things necessary to make this Third Supplemental
Indenture a valid supplement to the Indenture according to its terms and the
terms of the Indenture have been done.

                  NOW, THEREFORE, for and in consideration of the foregoing, the
parties hereto hereby enter into this Third 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.

                  The Additional Subsidiary Guarantors, by their signatures
below, agree to become Additional Subsidiary Guarantors under the Indenture with
the same force and effect as if originally named therein as Subsidiary
Guarantors. The Additional Subsidiary Guarantors hereby agree to all the terms
and provisions of the Indenture applicable to them as Additional Subsidiary
Guarantors thereunder and each reference to a "Subsidiary Guarantor" in the
Indenture shall be deemed to include the Additional Subsidiary Guarantors.

                  SECTION 3 Indenture remains in Full Force and Effect.

                  Except as expressly supplemented by this Third Supplemental
Indenture, the Indenture shall remain in full force and effect in accordance
with its terms.

                  SECTION 4 New York Law to Govern.

                  THIS THIRD 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
Third 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 Third Supplemental Indenture may be executed in
counterparts, each of which when taken together shall constitute one and the
same instrument. This Third

                                       2
<PAGE>

Supplemental Indenture shall become effective when the Trustee shall have
received a counterpart thereof that bears the signatures of each of the
Additional Subsidiary Guarantors.

                                       3
<PAGE>

                  IN WITNESS WHEREOF, the Company, the Additional Subsidiary
Guarantors, and the Trustee have duly executed this Third Supplemental Indenture
as of the day and year first above written.

                                         QUEST DIAGNOSTICS INCORPORATED,
                                         a Delaware corporation, as Company

                                         By:      /s/ Leo C. Farrenkopf, Jr.
                                            ------------------------------------
                                         Name:    Leo C. Farrenkopf, Jr.
                                         Title:   Deputy General Counsel,
                                                  Vice President and Secretary

                                         AMERICAN MEDICAL LABORATORIES,
                                          INCORPORATED
                                         AML INC.
                                         MEDICAL LABORATORIES CORPORATION
                                         APL HEALTHCARE GROUP, INC.
                                         APL PROPERTIES LIMITED COMPANY
                                         QUEST DIAGNOSTICS LLC (CT)
                                         QUEST DIAGNOSTICS LLC (MA),
                                         as Additional Subsidiary Guarantors

                                         By:      /s/ Joseph P. Manory
                                            ------------------------------------
                                         Name:    Joseph P. Manory
                                         Title:   Vice President and Treasurer

<PAGE>

                                         THE BANK OF NEW YORK, as Trustee

                                         By:     /s/ Marie Trimboli
                                            ------------------------------------
                                         Name:   Marie Trimboli
                                         Title:  Assistant Vice President

<PAGE>Exhibit 4.2

                  PREFERREDPLUS TRUST CERTIFICATES SERIES ELP-1

                         AMENDMENT TO SERIES SUPPLEMENT

                                     between

                         MERRILL LYNCH DEPOSITOR, INC.,

                                  as Depositor,

                                       and

                              THE BANK OF NEW YORK,

                     as Trustee and Securities Intermediary

                            Dated as of April 9, 2002

<PAGE>

                  AMENDMENT TO SERIES SUPPLEMENT, dated as of April 9, 2002 (the
"Amendment"), by and between MERRILL LYNCH DEPOSITOR INC., a Delaware
corporation, as Depositor, THE BANK OF NEW YORK, a New York corporation, as
Trustee and Securities Intermediary.

                                   WITNESSETH:

                  REFERENCE is made to the PREFERREDPLUS Trust Series ELP-1 (the
"Trust") created by the Depositor by executing and delivering the Series
Supplement dated as of March 25, 2002 (the "Series Supplement"), which
incorporates the terms of the Standard Terms for Trust Agreements, dated as of
February 20, 1998 (the "Standard Terms" and, together with such Series
Supplement, the "Trust Agreement"), by and between the Depositor and the Trustee
and Securities Intermediary, as modified by such Series Supplement;

                  WHEREAS, Section 3(b) of the Series Supplement permits the
Depositor to increase the amount of the Underlying Securities in the Trust and
the Trust to issue a corresponding number of additional Warrants subject to the
terms and conditions set forth in Section 3(b); and

                  WHEREAS, the Depositor, on March 25, 2002, deposited
$40,754,000 Underlying Securities into the Trust and now desires to increase the
number of Underlying Securities in the Trust from $40,754,000 to $50,754,000
and, therefore, increase the number of Class A and Class B Trust Certificates
issued by the Trust and the corresponding number of Warrants from 1,630,160 to
2,030,160;

                  NOW, THEREFORE, in consideration of the foregoing premises, it
is hereby agreed by and between the Depositor and the Trustee and Securities
Intermediary as follows:

Amendment of Series Supplement.

                  The Series Supplement is hereby amended as follows:

All references to "1,630,160" shall be replaced with "2,030,160"; and

All references to "$40,754,000" shall be replaced with "$50,754,000".

The following definition shall be added to Section 2 of the Series Supplement:

                           "Warrant" shall mean a Warrant Certificate as such
                           term is defined in the Warrant Agreement.

The introductory clause of Section 4 of the Series Supplement shall be amended
to read:

                           The Trustee hereby acknowledges receipt, with respect
                           to $40,754,000 of Underlying Securities, on or prior
                           to the Closing Date, and with respect to $10,000,000
                           of Underlying Securities, on or prior to April 9,
                           2002, of:

Additional Warrant Certificates. Simultaneously with the execution hereof, the
Depositor directs the Trustee, in the name and on behalf of the Trust, to enter
into additional Warrant

                                       2

<PAGE>

Certificates (as defined in the Warrant Agreement) evidencing the additional
Call Rights related to the increase in the amount of Underlying Securities.

References. All references to the Series Supplement or the Trust Agreement, in
any document executed in connection with the Series Supplement, shall be deemed
to refer to the Series Supplement or the Trust Agreement, as applicable, as
amended by this Amendment.

Miscellaneous.

Counterparts. This Amendment may be executed by the parties hereto in separate
counterparts, each of which when so executed and delivered shall be an original,
but all such counterparts shall together constitute but one and the same
instrument.

Amendments. Neither this Amendment nor any of the terms hereof may be
terminated, amended, supplemented, waived or modified orally, but only by an
instrument in writing signed by the party against which the enforcement of such
termination, amendment, supplement, waiver or modification is sought.

Governing Law. This Amendment shall in all respects be construed in accordance
with and governed by the laws of the State of New York,

Benefit and Binding Effect. The terms of this Amendment shall be binding upon,
and shall inure to the benefit of, the parties hereto and their successors and
permitted assigns.

                  Except as amended by this Amendment, the Series Supplement
remains in full force and effect.

                                       3

<PAGE>

                  IN WITNESS WHEREOF, the parties hereto have caused this
Amendment to be duly executed by their respective authorized officers as of the
date first written above.

                                            Merrill Lynch Depositor, Inc.,
                                            as Depositor

                                            By:      /s/ Barry Finkelstein
                                                 ------------------------------
                                            Name:    Barry N. Finkelstein
                                            Title:   President

                                            The Bank of New York,
                                            not in its individual capacity
                                            but as Trustee

                                            By:      /s/ Kevin Cremin
                                                 ---------------------
                                            Name:    Kevin Cremin
                                            Title:   Authorized Signatory

                                            The Bank of New York,
                                            as Securities Intermediary

                                                 By: /s/ Kevin Cremin
                                                     -----------------
                                                       Name: Kevin Cremin

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