The type and predicting factors of response to alpha interferon therapy have been studied in 26 patients with chronic non-A non-B hepatitis. Interferon was administered three times weekly during 6 months at a dose of 3 millions units/day. Eleven patients (42 percent) had serum alanine aminotransferase levels below 1.5 times the upper limit of normal range at the end of treatment. Only eight (31 percent) patients had persistent normalization of seric alanine aminotransferase value, 6 months after the end of the interferon treatment. The main factors involved in the response to therapy were age, apparent duration of the disease, and mode of contamination: patients who responded to interferon were younger, had a shorter duration of hepatitis and a parenterally transmitted disease.