To investigate the markers useful for evaluating the long-term efficacy of interferon (IFN) therapy, the quantity of hepatitis C virus (HCV) RNA and two anti-HCV antibody titers (anti-N14 and anti-C-100-3 antibody) in 21 chronic hepatitis C patients were determined. In all complete responders, a sustained clearance of the virus and reductions in the anti-HCV antibody titers were observed during and after therapy. In most of the temporary responders, reductions in the HCV RNA levels and in both anti-HCV antibody titers were observed temporarily during the therapy, and relapse followed. In nonresponders, although the HCV RNA levels and anti-N14 antibody titer tended to remain unchanged or increased during and after therapy, the anti-C-100-3 antibody titers showed no tendency. These results demonstrate that the monitoring of the HCV RNA level and anti-N14 antibody titer is clinically useful for following the patient's response to IFN therapy for chronic hepatitis C.