Pan-frying of hamburgers may increase the risk of cancer for those who eat them, a Washington University scientific research team has concluded.

In reporting its finding the group emphasized that the increased risk is as yet of "unknown magnitude." But it concluded that the dangers can be reduced to zero by broiling hamburgers under a heat source or cooking them in a microwave oven.

The research team is headed by a biologist, Dr. Barry Commoner, and affiliated with Washingyton University's Center for the Biology of Natural Systems. Its report will be presented tomorrow to the annual meeting of the American Society for Microbiology in Las Vegas.

Commoners research team found that ground beef hamburgers cooked on a hot metal surface at temperatures exceeding 300 degrees tend to form chemical substances called mutagens which "may represent a risk of cancer in people." In animal experiments 90 percent of the mutagens tested have been found to cause cancer in the test animals.

The researchers also discovered the same mutagens in beef bouillon cubes, beef broth and seasoning bases as well as beef-flavored sauce concentrates.

A mutagen is a substance that causes genetic change.

The new mutagens wer discovered after the St. Louis researchers noticed that bacteria placed in a certain medium for growth changed genetically at a higher rate than expected. They found that beef extract, found in many processed foods, which is used as part of the medium, was the cause of the excessive mutation. This led the researchers to test ground beef.

Tests showed that uncooked ground beef and ground beef broiled or cooked in a microwave oven do not contain the mutagens. That is why Commoner says there is no reason to give up eating hamburgers completely.

The potential risk from eating panfriend hamburgers is reduced by 90 percent if the meat is cooked only to the rare stage.

Although the report emphasizes that the risks are of unknown magnitude, Commoner said, "if action can be taken to reduce exposure without sacrificing the benefits of meat, then it should be done."

The researcher recommends broiling hamburgers and all other meats. "There's no reason to believe steak or any meat treated the same way would be any different," he said.

Commoner also said that charcoal-grilled meats are likely to produce the same mutagens. Earlier studies show that charcoal grilling also causes the fomation of known carcinogens on the charred surface of the meat.

The mutagens which were also found in the beef extracts appear to from when beef extracts appear to form when beef stock is heated and evaporated. Before that process takes place beef stock does not contain mutagenic substances.

Dr. Gilo Batta Gori, deputy director of the Division of Cancer Cause and Prevention at the National Cancer Institue, called Commoner's research "first-class science."

"These are the latest in a series of similar analyses starting with the findings of benso(a) pyrene (a cancer causing ingredient) in charcoal-broiled steaks," Gori said, "but that does not necessarily mean the mutagens are carcinogenis."

Work is under way to identify the mutagens so that their potential as carcinogens can be tested in laboratory animals.

Asked to comment on the implications of these findings for the world's largest fryer of hamburgers a spokesman for the McDonald's chain said: "I can't believe it . I haven't got the fain-test idea what we're going to do. We'll have to wait to see the research."

McDonald's and many other fast food franchises cook their hamburgers on a metal surface whileothers broil and some charcoal-broil.