In an unusual move, the head of the McDonald's Corporation has sent a memorandum to the company's 2,700 United States franchisees defending the introduction of the Arch Deluxe, saying that the sandwich was ''never intended to be a silver bullet.''

The memo by Edward H. Rensi, the company's president and chief executive, was intended to counter outside criticism of the introduction of the product, a hamburger with a supposedly more ''adult'' taste to attract older customers. Mr. Rensi also told franchisees that the company would officially introduce chicken and fish versions of the sandwich on Sept. 26, calling them ''a powerful new Deluxe line of sandwiches.''

In addition, Mr. Rensi tried to marshal market-research data in a defense of the Arch Deluxe to the franchisees, writing in summary: ''Only those who expected a miracle were disappointed.''

A spokesman for McDonald's declined to comment about the memo.

McDonald's officials said at the time of the introduction in May that they believed the Arch Deluxe would generate $1 billion in sales in its first year, and the company has said that sales of Arch Deluxe have been strong.