The clinical utility of a new JCC-2 enzyme-linked immunosorbent assay kit that detects and quantitates anti-hepatitis C virus (anti-HCV) core antibodies (anti-HCc) was investigated. Serum samples were obtained from 102 patients with various non-A, non-B liver diseases, including 19 cases of chronic hepatitis type C who had been treated with interferon (IFN). The results of the anti-JCC-2 assay were significantly correlated with serum HCV-RNA positivity. Patients who were HCV-RNA positive exhibited a high rate of positivity for anti-JCC-2 (72.2% in acute hepatitis, > 90% in chronic liver diseases). The geometric mean of the anti-JCC-2 titre was not significantly different among different stages of chronic liver disease (among CPH, CAH and LC). The anti-JCC-2 titre decreased gradually in cases that became HCV-RNA negative after IFN therapy. If HCV-RNA positivity recurred, the anti-JCC-2 titre increased, indicating that serial measurements of the anti-JCC-2 titre are useful for monitoring the antiviral effect of IFN treatment. These results suggest that quantification of anti-HCc by the anti-JCC-2 assay is superior to the semi-quantification of circulating HCV-RNA provided by monitoring of IFN therapy. Monitoring of HCV-RNA status using reverse transcription-nested polymerase chain reaction (RT-nested PCR) is possible, but it is technically demanding and too expensive for routine clinical use.