EXHIBIT 99.6

                          LETTER TO BROKERS, DEALERS,
                       COMMERCIAL BANKS, TRUST COMPANIES
                               AND OTHER NOMINEES

                               OFFER TO EXCHANGE
                       0.3256 OF A SHARE OF COMMON STOCK
          (INCLUDING THE ASSOCIATED RIGHT TO PURCHASE PREFERRED STOCK)

                                       OF

                         QUEST DIAGNOSTICS INCORPORATED

                                       OR

                                 $26.50 IN CASH

                                      FOR

                     EACH OUTSTANDING SHARE OF COMMON STOCK

                                       OF

                               UNILAB CORPORATION

SUBJECT, IN EACH CASE, TO THE PRORATION AND ELECTION PROCEDURES DESCRIBED IN THE
                                   PROSPECTUS
              AND THE RELATED LETTER OF ELECTION AND TRANSMITTAL.

  THE OFFER AND WITHDRAWAL RIGHTS WILL EXPIRE AT 12:00 MIDNIGHT, NEW YORK CITY
 TIME, ON MONDAY, JUNE 17, 2002, UNLESS THE OFFER IS EXTENDED. SHARES TENDERED
 PURSUANT TO THIS OFFER MAY BE WITHDRAWN AT ANY TIME PRIOR TO THE EXPIRATION OF
                                   THE OFFER.

                                                                    May 15, 2002

To Brokers, Dealers, Commercial Banks,
  Trust Companies and Other Nominees:

     We have been engaged by Quest Diagnostics Newco Incorporated ("Purchaser"),
a Delaware corporation and a wholly owned subsidiary of Quest Diagnostics
Incorporated, a Delaware corporation ("Quest Diagnostics"), to act as Dealer
Manager in connection with Purchaser's offer to exchange each outstanding share
of common stock, par value $.01 per share (the "Shares"), of Unilab Corporation,
a Delaware corporation (the "Company"), for (i) 0.3256 of a share of common
stock, par value $.01 per share, of Quest Diagnostics ("Quest Diagnostics
Shares"), or (ii) $26.50 in cash, without interest, at the election of the
holder of Shares and upon the terms and subject to the conditions set forth in
the Prospectus, dated May 15, 2002 (the "Prospectus"), and the related Letter of
Election and Transmittal (which, together with the Prospectus and any amendments
or supplements thereto, collectively constitute the "Offer") enclosed herewith.

     The Offer is being made pursuant to the Agreement and Plan of Merger, dated
as of April 2, 2002, and amended as of May 13, 2002 (as amended, the "Merger
Agreement"), among Quest Diagnostics, Purchaser and the Company. The Merger
Agreement provides, among other things, that following completion of the Offer
and the satisfaction or, if permissible, waiver of all conditions set forth in
the Merger Agreement, and in accordance with the Delaware General Corporation
Law ("Delaware Law"), the Company will be merged with and into Purchaser or,
depending on certain tax matters, Purchaser will be merged with and into the
Company (the "Merger"). At the effective time of the Merger, each


outstanding Share (other than Shares held in the Company's treasury or Shares
held by Quest Diagnostics, Purchaser or any wholly owned subsidiary of Quest
Diagnostics or Purchaser, or Shares held by any stockholder that has perfected
appraisal rights, if available) will be cancelled and converted into 0.3256 of a
Quest Diagnostics Share, upon the terms and conditions of the Merger Agreement.
Notwithstanding the amount, if any, of cash paid in the Offer, stockholders who
do not tender Shares in the Offer will not receive any cash consideration in the
Merger (except for cash, if any, that is paid in lieu of fractional Quest
Diagnostics Shares or following the exercise of appraisal rights, if
applicable). The Merger Agreement is more fully described in the section of the
Prospectus entitled "The Merger Agreement". Please furnish copies of the
enclosed materials to those of your clients for whose accounts you hold Shares
registered in your name or in the name of your nominee.

     THE OFFER IS CONDITIONED UPON, AMONG OTHER THINGS, (I) THERE HAVING BEEN
VALIDLY TENDERED AND NOT PROPERLY WITHDRAWN PRIOR TO THE EXPIRATION DATE (AS
DEFINED IN THE PROSPECTUS) OF THE OFFER THAT NUMBER OF SHARES WHICH CONSTITUTES
AT LEAST 50.1% OF THE AGGREGATE NUMBER OF OUTSTANDING SHARES ON A FULLY DILUTED
BASIS (AS THOUGH ALL OPTIONS EXERCISABLE FOR SHARES HAD BEEN CONVERTED,
EXERCISED OR EXCHANGED) AND (II) THE APPLICABLE WAITING PERIOD UNDER THE
HART-SCOTT-RODINO ANTITRUST IMPROVEMENTS ACT OF 1976, AS AMENDED, HAVING EXPIRED
OR BEEN TERMINATED PRIOR TO THE EXPIRATION OF THE OFFER. THE OFFER IS ALSO
SUBJECT TO OTHER CONDITIONS. SEE "THE OFFER - CONDITIONS TO THE OFFER" IN THE
PROSPECTUS.

     For your information and for forwarding to your clients for whom you hold
Shares registered in your name or in the name of your nominee, we are enclosing
the following documents:

          1.  The Prospectus, dated May 15, 2002;

          2.  The Letter of Election and Transmittal for use in accepting the
     Offer, tendering Shares, electing to receive either cash or Quest
     Diagnostics Shares in exchange for Shares and for the information of your
     clients. Facsimile copies of the Letter of Election and Transmittal with
     manual signature(s) may be used to tender Shares;

          3.  The Notice of Guaranteed Delivery to be used to accept the Offer
     if certificates evidencing Shares are not immediately available or if such
     certificates and all other required documents cannot be delivered to
     Computershare Trust Company of New York (the "Offer Exchange Agent") prior
     to the expiration date or if the procedures for book-entry transfer cannot
     be completed prior to the expiration date;

          4.  A letter to stockholders of the Company from Robert E. Whalen,
     Chairman, President and Chief Executive Officer of the Company, together
     with a Solicitation/Recommendation Statement on Schedule 14D-9 filed with
     the Securities and Exchange Commission by the Company;

          5.  A letter which may be sent to your clients for whose accounts you
     hold Shares registered in your name or in the name of your nominee, with
     space provided for obtaining such clients' instructions with regard to the
     Offer;

          6.  Guidelines for Certification of Taxpayer Identification Number on
     Substitute Form W-9; and

          7.  A return envelope addressed to the Offer Exchange Agent.

     WE URGE YOU TO CONTACT YOUR CLIENTS AS PROMPTLY AS POSSIBLE. PLEASE NOTE
THAT THE OFFER AND WITHDRAWAL RIGHTS EXPIRE AT 12:00 MIDNIGHT, NEW YORK CITY
TIME, ON MONDAY, JUNE 17, 2002, UNLESS THE OFFER IS EXTENDED.

     Upon the terms and subject to the conditions of the Offer (including, if
the Offer is extended or amended, the terms and conditions of any such extension
or amendment), Purchaser will accept for exchange Shares that are validly
tendered prior to the expiration of the Offer and not theretofore properly
withdrawn when, as and if Purchaser gives oral or written notice to the Offer
Exchange Agent of Purchaser's acceptance of such Shares for exchange pursuant to
the Offer. In all cases, payment for Shares accepted for exchange pursuant to
the Offer will be made only after timely receipt by the Offer Exchange Agent of
(i) certificates evidencing such Shares (or a confirmation of a book-entry
transfer of such Shares into the Offer Exchange Agent's account at the
Book-Entry Transfer Facility (as defined in the Prospectus), with any required
signature guarantees), (ii) a Letter of Election and Transmittal (or a manually
signed facsimile thereof) properly completed and duly executed with any required
signature guarantees or, in the case of a book-entry transfer, an agent's
message (as defined in the Prospectus) and (iii) all other required documents.

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     If holders of Shares wish to tender, but it is impracticable for them to
forward their certificates or other required documents or to complete the
procedures for delivery by book-entry transfer prior to the expiration date of
the Offer, a tender may be effected by following the guaranteed delivery
procedures described in "The Offer -- Procedures for Tendering and
Electing -- Guaranteed Delivery" in the Prospectus.

     Neither Quest Diagnostics nor Purchaser will pay any fees or commissions to
any broker, dealer or other person (other than the Dealer Manager, the Offer
Exchange Agent (as described in the Prospectus) and the Information Agent (as
defined below)) in connection with the solicitation of tenders of Shares
pursuant to the Offer. However, Purchaser will, upon request, reimburse you for
customary mailing and handling expenses incurred by you in forwarding any of the
enclosed materials to your clients. Purchaser will pay or cause to be paid any
stock transfer taxes payable with respect to the transfer of Shares to it,
except as otherwise provided in the Letter of Election and Transmittal.

     Any inquiries you may have with respect to the Offer should be addressed to
Merrill Lynch & Co. or Georgeson Shareholder Communications Inc. (the
"Information Agent") at their respective addresses and telephone numbers set
forth on the back cover page of the Prospectus.

     Additional copies of the enclosed material may be obtained from the
Information Agent or the Dealer Manager, at their respective addresses and
telephone numbers set forth on the back of the Prospectus.

                                         Very truly yours,

                                         MERRILL LYNCH & CO.

     NOTHING CONTAINED HEREIN OR IN THE ENCLOSED DOCUMENTS SHALL CONSTITUTE YOU
OR ANY OTHER PERSON THE AGENT OF QUEST DIAGNOSTICS, PURCHASER, THE COMPANY, THE
DEALER MANAGER, THE INFORMATION AGENT OR THE OFFER EXCHANGE AGENT, OR OF ANY
AFFILIATE OF ANY OF THE FOREGOING, OR AUTHORIZE YOU OR ANY OTHER PERSON TO USE
ANY DOCUMENT OR TO MAKE ANY STATEMENT ON BEHALF OF ANY OF THE FOREGOING IN
CONNECTION WITH THE OFFER OTHER THAN THE ENCLOSED DOCUMENTS AND THE STATEMENTS
CONTAINED THEREIN.

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