Scientists believe they have found a virus responsible for a third form of hepatitis -- a discovery that promises to eliminate 90 percent of the remaining hepatitis found in blood stored in U.S. blood banks.

Hepatitis A and B already are virtually eliminated from blood banks, said researcher Belinda Seto of the Food and Drug Administration, but 1 to 2 percent of the blood donated each year contains another form of hepatitis. Almost all of it is believed to be the newly discovered form. Dubbed hepatitis C, the serious and often fatal liver disease affects as many as 120,000 Americans a year, Seto said.

A team led by Dr. Robert J. Gerety has been searching since 1972 for a third kind of hepatitis -- one that is neither type A nor type B. Their discovery points to a retrovirus, a kind of virus that is known to cause only two other diseases in humans -- a type of leukemia and acquired immune deficiency syndrome (AIDS).

Like hepatitis B, the new version is transmitted through blood or other body fluids, Seto said. The next step in research, she said, is to perfect an existing test for non-A, non-B hepatitis based on the new knowledge and screen the blood supply -- as is now done for the other forms of hepatitis.

A vaccine, such as the one that already exists for hepatitis B, could be developed in a matter "of years," she said.