Scientists say they have the first hard evidence that a previously unknown infectious agent, probably a virus, is repsonsible for most of the hepatitis resulting from blood transfusions.

Government scientists said yesterday that the agent, not recognized as a separate disease entity until recently, has become the most prevalent cause of post-transfusion hepatitis.

This side effect of transfusions, which still affects an estimated 1508000 to 200,000 people a year, has declined 90 percent in the past 10 years because of a nationwide effort to eliminate commerical blood collecting, the chief source of viruses previously known to cause the liver ailment.

But dispite these efforts, post-tranfusion hepatitis persists, and scientists say the new agent is the culprit.

In the past, the principal cause of post-transfusion hepatitis, once known as serum hepatitis, was a virus called type B. Another less severe virus, called type A, also is implicated.

Tests developed in recent years to screen for evidence of these viruses eliminated them as the cause of most of the remaining post-transfusion hepatitis, indicating that another agent or group of agents was involved. Because scientists don't know what the agent is, they named the new form of the disease "non-A, non-B hepatitis" for what it isn't.

Dr. Harvey J. Alter of the National Institutes of Health told a news conference, "we assume it is another virus, but we have no proof, even though it looks and acts like a virus. And there are indictions that it could be more than one agent.

Alter and Dr. Edward Tabor of the Food and Drug Administration found in independent studies that the new agent is infectious and transmissible to chimpanzees from human blood.

"This is the first concrete evidence that a virus or virus-like agent is involved," Tabor said. "Earlier evidence was circumstantial."

Tabor said this work gives researchers an important animal model to use in trying to identify the agents and develop a screening test for the disease.

Scientists say that non-A, non-B hepatistis has a long incubation period: 5 to 12 weeks after transfusion, signs of it show up. Like type B hepatitis, the disease occasionally can be fatal, and there is evidence that people can carry the disease and transmit it without showing symptoms.

About 20 percent of viral hepatitis victims can become very ill with the liver inflammation, and it can lead to liver failure and the death of some tissue in severe cases. Doctors can only treat the symptoms, as there is no cure, but most patients recover after the disease has run its course.

Federal health authorities say about 3 million people receive blood transfusions each year in the United States and about 7 to 10 percent of them will get post-transfusion hepatitis.