Serum interferon activity was measured in 40 patients with acute viral hepatitis A, B and non-A-non-B during the acute stage of the disease and correlated with the severity, the long-term outcome and the viral etiology of the disease. Patients with alcoholic hepatitis, patients with an influenza-like illness and healthy volunteers served as controls. 80% of all patients with virus hepatitis revealed no measureable or only borderline interferon activity in their serum. No correlation was found with severity and long-term outcome of the disease, but patients with virus hepatitis A showed a stronger interferon induction than patients with hepatitis B and non-A-non-B. Further investigations of the interferon system in patients with virus hepatitis might help to improve our understanding of the different forms of the disease. The data presently available, however, do not permit as yet to define the value of interferon in the treatment of severe forms of acute virus hepatitis.