Twenty-four patients with chronic non-A non-B hepatitis were randomly assigned to receive either human fibroblast interferon (HuIFN-beta) at doses of 1 or 3 million international units (MIU) per day for 4 or 12 weeks (12 patients) or to receive no therapy (12 patients), and were then compared with 5 patients with chronic type B hepatitis who were treated with HuIFN-beta. Elevated serum aminotransferase levels decreased more rapidly during the treatment of chronic non-A non-B hepatitis than of chronic hepatitis B. Variations in serum aminotransferases were not observed in any of the untreated chronic non-A non-B hepatitis patients. In 3 of the 9 patients with chronic non-A non-B hepatitis who responded to HuIFN-beta therapy, serum aminotransferase levels remained normal 15, 21 and 31 months after therapy was discontinued; liver biopsy specimens obtained after therapy from 2 patients showed marked histological improvement. In the six other patients aminotransferase activity levels became again elevated following cessation of interferon therapy. No response to HuIFN-beta was seen in the remaining 3 patients with chronic non-A non-B hepatitis.