EXHIBIT 23 INDEPENDENT AUDITORS' CONSENT We consent to the incorporation by reference in the previously filed Registration Statements of Marsh & McLennan Companies, Inc. on Form S-8 (Registration Statement File Nos. 2-58660, 2-65096, 2-82938, 33-21566, 33-32880, 33-48803, 33-48804, 33-48807, 33-54349, 33-59603, 33-63389, 333-35739, 333-35741, 333-29627 and 333-51141) and, the previously filed Registration Statements on Form S-3 (Registration Statement Nos. 333-25069, 333-28201, 333-41021, 333-48707 and 333-67543) of our reports dated March 5, 1999 appearing in, and incorporated by reference in, this Annual Report on Form 10-K of Marsh & McLennan Companies, Inc. for the year ended December 31, 1998. /s/ Deloitte & Touche LLP New York, New York March 26, 1999