The 2'-5' oligoadenylate synthetase (2-5 AS) activity of peripheral blood mononuclear cells and serum was measured in 23 patients with chronic non-A, non-B hepatitis during interferon therapy, 16 of whom were found to have antibody to hepatitis C virus (anti-HCV). Patients received a daily dose of either 1 million, 3 million or 6 million units of human interferon-alpha or -beta for 4 to 6 weeks. Before treatment, the 2-5 AS activity was not significantly different from that in normal control subjects or patients with chronic hepatitis B. However, during treatment the 2-5 AS activity increased 2- to 41-fold from the initial level. Alanine aminotransferase (ALT) levels normalized promptly after the start of treatment in 15 (65.2%) of the 23 patients, but remained elevated in the remaining 8 (34.8%). Six (40%) of the 15 patients showed consistently normal ALT levels for 6 to 30 months after the end of treatment. There was no significant difference between the responders and non-responders in the pattern of change of 2-5 AS activity, but pretreatment activity levels in peripheral blood mononuclear cells were significantly higher (P less than 0.001) in the patients whose ALT levels did not normalize during treatment. The frequency of patients with a positive anti-HCV was significantly higher (P less than 0.05) in the group in which ALT levels normalized. Therefore, these findings suggest that the pretreatment 2-5 AS activity and the detection of anti-HCV may be useful parameters for predicting the response to interferon therapy.