Amounts of interferon were measured in sera from 59 patients with acute viral hepatitis and 49 patients with chronic hepatitis B and compared to those from patients with nonviral liver disease or influenza, and from healthy controls. In all patients with acute and chronic viral hepatitis, no serum interferon could be detected, confirming data from earlier studies of acute viral hepatitis in which no circulating interferon was found. Our results disprove the view that the amounts of serum interferon, detected at the time of the acute clinical illness, may be a determinant of outcome.