Anti-P1P4 core antibody, derived from a Japanese hepatitis C virus clone, was evaluated clinicopathologically in serum samples from 40 blood donors positive for anti-HCV antibody by 2nd generation assay and in 37 patients with HCV chronic hepatitis treated with interferon. The presence of anti-P1P4 antibody was highly correlated with the presence of HCV-RNA in the blood donors. In the patients with chronic hepatitis, more than a 50% reduction in P1P4 antibody titer after interferon therapy suggested the disappearance of HCV-RNA from the blood. Thus, anti-P1P4 antibody was useful in evaluating the virological effects of interferon therapy. However, clinically and pathologically, the titer of P1P4 antibody did not indicate the grade of liver inflammation.