The activity of antibodies to hepatitis C virus (anti-HCV) was investigated in 80 patients with chronic non-A, non-B liver diseases. Anti-HCV antibodies were positive in 82.5% (66/80), and the titers were 1 for 18 patients, 2 for 40 and 3 for 8, respectively. The frequency of anti-HCV was significantly lower in patients with chronic persistent hepatitis (8/13, 61.5%) than in those with chronic active hepatitis (42/49, 85.7%) (P less than 0.05). There was no significant difference in the distribution of anti-HCV titers among the different stages of hepatitis. There was no correlation between anti-HCV titer and histology activity index score in chronic hepatitis. Activity of anti-HCV decreased more frequently in the patients who responded to interferon alpha (IFN alpha) therapy (8/22, 36.4%) than in those who did not (0/9, 0%) (P less than 0.05). These results indicate that anti-HCV activity does not correlate with the activity or disease stage of chronic hepatitis, but that anti-HCV activity decrease more frequently during IFN alpha treatment in patients who responded to IFN alpha therapy.