In interferon therapy for chronic hepatitis type C, we assayed C100-3 and N14 antibodies, and also determined IgM-C100-3 and IgM-N14 antibodies as well as HCV-RNA. N14 antibody was assayed by an end point titer method. In patients responding to interferon therapy, C100-3 and N14 antibody titers were decreased and became negative. In patients irresponsive to interferon therapy, C100-3 antibody titers were hardly changed, while N14 antibody titers were markedly increased. IgM-C100-3 and IgM-N14 antibody levels were significantly higher in the patients non-responding to interferon therapy than in those responding to interferon therapy. The daily assays of C100-3 and N14 antibody titers are considered useful in judging the effectiveness of interferon against chronic hepatitis type C.