-------------------------------------------------------------------------------- SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 -------------- SCHEDULE TO Tender Offer Statement Under Section 14(d)(1) or 13(e)(1) of the Securities Exchange Act of 1934 (Amendment No. 23) -------------- UNILAB CORPORATION (Name of Subject Company) -------------- QUEST DIAGNOSTICS NEWCO INCORPORATED QUEST DIAGNOSTICS INCORPORATED Offeror (Names of Filing Persons (identifying status as offeror, issuer or other person)) Common Stock, Par Value $.01 Per Share (Title of Class of Securities) 904763208 (CUSIP Number of Class of Securities) -------------- Leo C. Farrenkopf, Jr. Quest Diagnostics Incorporated Vice President and Secretary One Malcolm Avenue Teterboro, New Jersey 07608 (201) 393-5000 (Name, Address and Telephone Number of Persons Authorized to Receive Notices and Communications on Behalf of Filing Persons) -------------- Copy to: Stephen T. Giove Clare O'Brien Shearman & Sterling 599 Lexington Avenue New York, New York 10022 (212) 848-4000 [ ] Check the box if the filing relates solely to preliminary communications made before the commencement of a tender offer. Check the appropriate boxes to designate any transactions to which the statement relates: [X] third-party tender offer subject to Rule 14d-1. [ ] issuer tender offer subject to Rule 13e-4. [ ] going-private transaction subject to Rule 13e-3. [ ] amendment to Schedule 13D under Rule 13d-2. Check the following box if the filing is a final amendment reporting the results of the tender offer: [ ] This Amendment No. 23 amends and supplements the Tender Offer Statement on Schedule TO (this "Schedule TO") filed by Quest Diagnostics Incorporated, a Delaware corporation ("Quest Diagnostics"), and Quest Diagnostics Newco Incorporated ("Purchaser"), a Delaware corporation and a wholly owned subsidiary of Quest Diagnostics on May 15, 2002, as amended by Amendment No. 1 on June 7, 2002, Amendment No. 2 on June 24, 2002, Amendment No. 3 on July 2, 2002, Amendment No. 4 on July 11, 2002, Amendment No. 5 on July 17, 2002, Amendment No. 6 on July 19, 2002, Amendment No. 7 on July 31, 2002, Amendment No. 8 on August 14, 2002, Amendment No. 9 on August 28, 2002, Amendment No. 10 on September 12, 2002, Amendment No. 11 on September 20, 2002, Amendment No. 12 on September 26, 2002, Amendment No. 13 on October 10, 2002, Amendment No. 14 on October 24, 2002, Amendment No. 15 on November 7, 2002, Amendment No. 16 on November 21, 2002, Amendment No. 17 on December 2, 2002, Amendment No. 18 on December 6, 2002, Amendment No. 19 on December 20, 2002, Amendment No. 20 on January 6, 2003, Amendment No. 21 on January 7, 2003 and Amendment No. 22 on January 17, 2003. This Schedule TO relates to the offer by Quest Diagnostics, through Purchaser, to exchange all outstanding shares of common stock, par value $.01 per share ("Shares"), of Unilab Corporation, a Delaware corporation (the "Company"), for, at the election of the holder thereof, (i) 0.3424 of a share of common stock, par value $.01 per share, of Quest Diagnostics ("Quest Diagnostics Shares") or (ii) $19.10 in cash, without interest, upon the terms and subject to the conditions set forth in the Prospectus, dated May 15, 2002, as amended on June 7, 2002, June 24, 2002 and July 11, 2002 (the "Original Prospectus"), as amended and supplemented by Prospectus Supplement No. 1, dated January 21, 2003 ("Prospectus Supplement No. 1" and together with the Original Prospectus, the "Prospectus"), and in the related Letter of Election and Transmittal (together, the Prospectus and the Letter of Election and Transmittal, with any amendments or supplements thereto, collectively constitute the "Offer"). The information set forth in the Prospectus and the related Letter of Election and Transmittal is incorporated herein by reference in response to Items 1-11 of this Schedule TO. Item 11. Additional Information Quest Diagnostics filed Post-Effective Amendment No. 1 (the "Post-Effective Amendment") to the Registration Statement on Form S-4 relating to the Offer, amending and supplementing the Original Prospectus, and such Prospectus is attached hereto as Exhibit (a)(24). Quest Diagnostics also issued a press release extending the expiration date of the Offer until Monday, February 3, 2003, and such press release is attached hereto as Exhibit (a)(25). The information set forth in the press release is incorporated in its entirety herein by reference. Item 12. Material to Be Filed as Exhibits (a)(24) Prospectus dated January 21, 2003 (incorporated by reference to Post-Effective Amendment No. 1 to Quest Diagnostics' Registration Statement on Form S-4, filed on January 21, 2003). (a)(25) Press Release dated January 21, 2003 (incorporated herein by reference to Quest Diagnostics' Form 425 filed on January 21, 2003). 2 Item 13. Information Required by Schedule 13E-3 Not applicable. 3 After due inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct. Dated: January 21, 2003 QUEST DIAGNOSTICS INCORPORATED By: /s/ Leo C. Farrenkopf, Jr. ------------------------------------ Name: Leo C. Farrenkopf, Jr. Title: Vice President and Secretary 4 After due inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct. Dated: January 21, 2003 QUEST DIAGNOSTICS NEWCO INCORPORATED By: /s/ Leo C. Farrenkopf, Jr. ------------------------------------ Name: Leo C. Farrenkopf, Jr. Title: Vice President and Secretary 5 EXHIBIT INDEX Exhibit No. (a)(24) Prospectus dated January 21, 2003 (incorporated by reference to Post-Effective Amendment No. 1 to Quest Diagnostics' Registration Statement on Form S-4, filed on January 21, 2003). (a)(25) Press Release dated January 21, 2003 (incorporated herein by reference to Quest Diagnostics' Form 425 filed on January 21, 2003). 6