The purpose of this study was to determine whether the response of serum alanine aminotransferase (ALT) to recombinant alpha-interferon was related to the presence or absence of antibodies to hepatitis C virus (anti-HCV) in patients with chronic non-A,non-B hepatitis. A group of 65 patients with chronic non-A, non-B hepatitis was studied. The source of contamination was blood transfusion or administration of blood products in 32, intravenous drug addiction in 14 and unknown in 19. The patients received 1, 3 or 5 MU of recombinant alpha-interferon, three times a week, for 6 months. A complete response was defined as normal ALT by the end of recombinant alpha-interferon treatment. Sera collected before treatment were tested for anti-HCV with an enzyme immunoassay. The overall percentage of anti-HCV positive patients in the study group was 75%. There was no difference between the anti-HCV positive and the anti-HCV negative patients before the treatment with respect to age, sex ratio, source of contamination, serum albumin, prothrombin, bilirubin, ALT or prevalence of cirrhosis. In the anti-HCV positive and the anti-HCV negative groups, there was no difference in the proportion of patients receiving the 1, 3 or 5 MU dosage. The percentage of patients with complete response was not different in anti-HCV positive patients (48%) and in anti-HCV negative patients (50%). There was also no difference in the kinetics of the decrease of mean serum ALT levels between the two groups.(ABSTRACT TRUNCATED AT 250 WORDS)