McDonald's will soon introduce nationally a low-fat hamburger called McLean Deluxe, according to published reports.

Analysts said it could start a trend, and at least one health activist hailed the menu addition.The McLean Deluxe, which underwent test-marketing beginning in November in Harrisburg, Pa., and eventually four other cities, will be available nationally April 26, The New York Times said Wednesday. It cited unidentified McDonald's franchises.

McDonald's Corp. refused to confirm the reports but scheduled a news conference today at its test kitchens in this Chicago suburb to announce the national introduction of what it described as "a major new product."

Low-fat burgers have been offered before, but they relied on soy substitute for meat.

Meat in the McLean is made from a new process using carrageenan, a common food additive made from seaweed, The Wall Street Journal reported Wednesday. The process devised at Auburn University in Alabama uses carrageenan to bind the beef, allowing water to replace fat.

The McLean Deluxe, including condiments, contains about 310 calories and derives 29 percent of its calories from fat, the Times reported. That compares with 410 calories for the Quarter Pounder, with 44 percent of its calories as fat.

The cost will probably be as much as a McD.L.T., which varies widely around the country, the papers said.

Phil Sokolof, an anti-cholesterol crusader who in the past assailed the fat content of McDonald's fare, praised the prospect of low-fat burgers as a "dramatic breakthrough."

"I'm elated that McDonald's is going national with a truly low-fat hamburger," said Sokolof, president of the National Heart Savers Association.

"This is what we have been striving for," he said in a statement. "This hamburger has the potential to be a major factor in lowering the fat intake of the American people."

Groups such as the American Heart Association, the National Cancer Institute and the American Dietetic Association recommend that fat should comprise less than 30 percent of calories consumed in a serving of food.

Analysts say the McLean Deluxe could improve the nutritional image of McDonald's food and start an industry trend.

Michael G. Mueller, an analyst at Montgomery Securities in San Francisco, said the low-fat burger "would help convince McDonald's customers that the company is in the forefront of health issues."