CAMBRIDGE, Mass. (AP) _ Youngsters who love milk should stick to the fat-free variety, because dairy products have surpassed meat as the top source of unhealthy saturated fat in teen-agers' diets, a researcher says.

Studies by Dr. Curtis Ellison at two elite New England prep schools found that 35 percent of students' daily intake of saturated fat comes from milk, cheese, ice cream and other dairy food.

Preppies are not unique in what they eat, according to Ellison, who said at least two other unpublished studies have reached similar conclusions.

''The studies show that the main source of saturated fat is not meat anymore,'' he said. ''It is milk and other dairy products.''

Eating too much saturated fat raises cholesterol levels in the bloodstream. This clogs the arteries that feed the heart. Decades of bad eating eventually can result in a heart attack, but many experts believe that the damage begins early in life.

Ellison measured the sources of salt and saturated fat in the diets of students at Phillips Exeter Academy in New Hampshire and Phillips Academy in Andover, Mass., as part of a study of dietary changes.

He found that after dairy products, meat, fish, poultry and eggs provided 28 percent of the students' saturated fat; baked goods and cereals 16 percent; ketchup, gravy and other ''food adjuncts'' 7 percent; fruits and vegetables 7 percent, snacks 6 percent, and everything else 1 percent.

Ellison said that many people shy away from red meat because of its fat content, but they sometimes substitute kinds of cheese, such as chedder cheese, that contain far higher levels of saturated fat.

''Kids love milk,'' he said. ''Some of them drink two liters a day, and they grow up thinking that it's a good thing to have. We are not trying to take their milk away. Just take the fat out of it.''

Ellison presented his data Tuesday at a program on school lunches held at Massachusetts Institute of Technology's Clinical Research Center.

Ellison is among health experts who believe youngsters should cut down on saturated fat and salt to establish good eating habits and prevent coronary artery disease and high blood pressure as they get older.

Salt aggravates high blood pressure, and his prep school survey found that breakfast cereal and baked goods provide about 30 percent of the salt in teen- agers' diets. Meats add 21 percent, dairy products 16 percent, food adjuncts 12 percent, fruits and vegetables 9 percent, snack foods 6 percent, table salt less than 1 percent and all else 5 percent.

In dietary experiments at the academies, Ellison found that by substituting low-salt recipes in the cafeterias, cooks could make food that the students would eat but still reduce their salt consumption by 15 percent to 20 percent. This also reduced the youngsters' average blood pressure by about 2 points.

In another experiment, cooks substituted polyunsaturated fat for saturated fat in recipes. This reduced saturated fat consumption from 13 percent of total calories to 10.6 percent. However, because of problems measuring the youngster's blood cholesterol levels, it was unclear what effect this had on their cardiovascular health.

Ellison said the average cholesterol levels of U.S. youngsters is between 160 and 170. The ideal is 110, although this is probably not obtainable if they follow a typical American diet.

''Our target is to get all children in the United States down around 140 over the next 10 years,'' he said. Even with cholesterol levels this low, some of them will still have cholesterol over 200 after they reach adulthood.