Researchers at the Food and Drug Administration say they have found the first evidence pointing to the cause of the most common type of hepatitis transmitted in blood transfusions, evidence that could lead to an effective screening test to protect blood supplies from the disease.

Dr. Robert J. Gerety, head of the FDA's hepatitis branch, said yesterday that -- to his surprise -- the new studies indicate that the culprit appears to be a virus belonging to the same family as the recently discovered virus responsible for the deadly immune system disease AIDS.

The hepatitis discovery, which will require confirmation by other laboratories before it is accepted, is a major coup by a government agency better known for regulation than research. The research, conducted in cooperation with scientists from Sweden and the National Institutes of Health, will be described in the Oct. 27 issue of the British medical journal Lancet.

The discovery comes after a highly competitive search by scientists around the world for the cause of an elusive form of hepatitis that was recognized only a decade ago.

Until then, it was thought that there were only two types of hepatitis. Hepatitis A, a less serious acute illness, was spread through fecally contaminated food or water. The more severe hepatitis B, which caused short-term illness as well as long-term liver disease, was known to be spread through blood transfusions as well as close personal contact.

But after the isolation of the hepatitis B virus in 1970 and the subsequent development of a screening test to eliminate it from the blood supply, researchers found that another type of hepatitis was still transmitting the disease through blood transfusions.

Known as the "non-A, non-B" form of hepatitis, it is thought to be responsible for at least 120,000 illnesses and about 1,200 deaths in the United States each year. A high proportion of the victims also go on to suffer chronic infections that can lead to lasting liver damage.

Gerety said non-A, non-B hepatitis accounts for about 90 percent of the estimated 100,000 cases of hepatitis that are still transmitted annually through blood transfusions.

The FDA researcher believes that he has sufficient evidence to move ahead over the next year on commercial development of a blood screening test that could prevent the disease from spreading that way. Work that could lead to a possible vaccine to prevent the disease can also begin.

Further work is needed to isolate and grow the specific virus that is responsible. But Gerety says he has been able to pinpoint several crucial characteristics of the virus, and his staff has had a glimpse of the virus itself in some liver samples. "I think this is the major agent," he said.

"We believe we have the cause, but we want further evidence. . . . I think this will be accepted as a major finding," said the FDA commissioner, Dr. Frank Young, who is trying to reemphasize scientific research at the agency he joined this summer.

One outside researcher described the discovery as "extremely exciting" but cautioned that it is "very preliminary and needs to be confirmed." He added that "it's the most fascinating thing to come down the line. If it is true, it's really going to set the world on its ear."

The new non-A, non-B hepatitis infectious agent appears to be a member of the so-called retrovirus family, a group of viruses long recognized in animals that has recently been linked with human diseases. Work by Dr. Robert Gallo, a National Cancer Institute researcher, has implicated various forms of one kind of them, Human T-Cell Leukemia Viruses, as a cause of a human blood cancer and the deadly acquired immune deficiency syndrome.

The retroviruses, whose genetic material is RNA or ribonucleic acid, copy themselves in the cell in an unusual fashion and can become a permanent part of their victim's own DNA genes.

Gerety said that signs of a specific enzyme that controls the reproduction of retroviruses was detected in four specimens of blood and two blood plasma products known to be able to transmit non-A, non-B hepatitis. In addition, the same type of virus pattern was found in blood taken from 12 patients known to have acute or chronic forms of the disease. It was seldom detected in a control group of healthy individuals, some of whom were at risk of getting the disease.

Gerety said the hepatitis discovery was inspired by AIDS blood research under way in his laboratory and carried out by Dr. Belinda Seto of FDA, in cooperation with her husband, Dr. William Coleman Jr. of the National Institute of Arthritis, Diabetes and Digestive and Kidney Diseases. Blood samples were also supplied by a Swedish researcher, Dr. Sten Iwarson.