Fulminant hepatic failure is a usually fatal complication of hepatitis. Little is known about the contribution of host factors, environment, viral mutation or epidemiologic conditions to its occurrence. During a tenweek period when fulminant hepatitis occurred in ten of 48 reported cases of viral hepatitis in a single rural county, a detailed analysis of these cases was undertaken. MATERIALS AND METHODS Between Dec 28, 1972, and March 14, 1973, seven patients with acute fulminant hepatitis were treated at the UCLA Center for the Health Sciences and the Wadsworth Veterans Administration Hospital, Los Angeles. Two other patients were treated at Kern County General Hospital and one was treated at Memorial Hospital of Bakersfield, Calif. These three patients died as a result of their illness. Hepatitis B Antigen (HBAg) subtyping by rheophoresis was carried out on all patients who were HBAg positive.1 Blood samples were obtained from three patients in Bakersfield