Hepatitis C virus is the most frequent cause of chronic non-A, non-B hepatitis, and the antibodies to structural and nonstructural proteins encoded by viral genome have been suggested to be markers of ongoing HCV infection. We studied the behavior of these antibodies during interferon therapy in 18 patients with chronic hepatitis C and also during a follow-up period of at least four years. A significant decrease of anti-HCV titer was found only in patients who had shown positive response to therapy and all of them were anti-HCV negative at the end of follow-up. Analysis by recombinant immunoblotting assay showed that only anti-c100 were affected by interferon therapy, whereas anti-c22 and anti-c33 were not modified. Using polymerase chain reaction to detect small amounts of HCV genome in serum, we could confirm that the behavior of HCV-RNA during and after interferon therapy is similar to that of anti-HCV and the loss of anti-c100 seems to be closely related to HCV-RNA disappearance from serum. Our patients with chronic hepatitis C were found to be of type 1b and 2, according to the recent score of Simmonds, and the clearance of serum HCV-RNA during treatment and its sustained negative status are closely related to genotype 2 and to long-term positive response to interferon.