Merck & Co. (MRK) 10-Q 2007 Q1 Quarterly report

Exhibit 15

April 26, 2007

To the Shareholders and Board of Directors of Schering-Plough Corporation:

We have reviewed, in accordance with the standards of the Public Company Accounting Oversight Board (United States), the unaudited interim financial information of Schering-Plough Corporation and subsidiaries (the “Corporation”) for the three-month periods ended March 31, 2007 and 2006, as indicated in our report dated April 26, 2007 (which report included an explanatory paragraph regarding the Corporation’s adoption of Statement of Financial Accounting Standards (“SFAS”) No. 123 (Revised 2004), “Share-Based Payment”, SFAS No. 158, “Employers’ Accounting for Defined Benefit Pension and Other Postretirement Plans”, and FASB Interpretation No. 48, “Accounting for Uncertainty in Income Taxes”); because we did not perform an audit, we expressed no opinion on that information.

We are aware that our report referred to above, which is included in your Quarterly Report onForm 10-Q for the quarter ended March 31, 2007, is incorporated by reference in Registration StatementsNo. 2-83963,No. 33-50606,No. 333-30331,No. 333-87077,No. 333-91440,No. 333-104714,No. 333-105567,No. 333-105568,No. 333-112421,No. 333-121089 andNo. 333-134281 onForm S-8, Post Effective Amendment No. 1 to Registration StatementNo. 2-84723 onForm S-8, Post Effective Amendment No. 1 to Registration StatementNo. 333-105567 onForm S-8 and Registration StatementsNo. 333-12909,No. 333-30355, andNo. 333-113222 onForm S-3.

We also are aware that the aforementioned report, pursuant to Rule 436(c) under the Securities Act of 1933, is not considered a part of the Registration Statements prepared or certified by an accountant or a report prepared or certified by an accountant within the meaning of Sections 7 and 11 of that Act.

/s/  Deloitte & Touche LLP

Parsippany, New Jersey