When pressed on this point, Michael Houghton, the Chiron scientist who led the research team, acknowledged that the company's data fail to meet the rigorous standards that are usually required to prove that a microorganism causes a disease. Chiron has not, for example, grown the virus in the laboratory or used the identified virus to reproduce the disease by inoculating test animals.

"It will allow a direct diagnosis of the disease," said Lacy Overby, a former Chiron vice president and a leading authority on hepatitis blood tests. "I've been through all the other false alarms and my opinion is this is it (the real thing)." "It is not ironclad proof, but it is by far the strongest evidence to date (that the virus has been found)," said Dr. Harold Varmus of the University of California Medical Center in San Francisco, who heard a presentation of some of the data at a university seminar Monday.

"The gamble paid off," Houghton said in a telephone interview. In the spring of 1987, after five years of work, the Chiron researchers identified genes that appeared to be part of the non-A, non-B hepatitis virus. The proteins produced by these genes reacted specifically with antibodies in the patients' serum.