In 1989, the Chiron group developed an enzyme immunoassay system (C100) for detecting antibodies to hepatitis C virus (anti-HCV). In our examinations, the positive rate was 65.3% of the total number of patients (199) with non-A, non-B (NANB) liver disease. Additionally, a specific ELISA system (KCL-163) was developed by Kaketsuken (Kumamoto, Japan) based on synthetic peptides corresponding to an HCV genome of a Japanese isolate. In this study, we measured antibodies to HCV by KCL-163, and compared the results of KCL-163 with that of C100 in patients with various liver diseases. Our emphasis was on the discrepancies between the results of the two types of ELISA. We concluded that it was not sufficient to diagnose hepatitis C only by assay systems based on the C100-3 region of an HCV genome, and that KCL-163 was superior to C100 in its specificity and sensitivity in the diagnosis of hepatitis C.