An ELISA using N14E plus #19-#40, a synthetic peptide of the HCV core for the purpose of improving diagnosis of the HCV infection was studied to evaluate its usefulness in comparison with the tests of C100-3 antibody, N14 antibody and HCV EIA II. N14E plus #19-#40 is made up of 34 amino acids, containing the first two hydrophilic regions of HCV core forming epitopes. By using N14E plus #19-#40 antibody, HCV-antibody was detected in 94.4% of chronic hepatitis, 80.6% of liver cirrhosis and 87.5% of hepatocellular carcinoma of non-A non-B chronic liver diseases. This assay seems to be more sensitive than the assays of C100-3 antibody and N14 as far as tested for our patients in this district. In chronic liver diseases, 5.6% (5/90) of patients negative for HCV EIA II were positive to anti-N14E plus #19-#40; one of these 5 patients was HCV RNA positive. Anti-N14E plus #19-#40 was detected in 57.5% out of 40 paid blood donors with elevated ALT higher than 50IU and negative for HBsAg, C100-3 and N14, but positive for HCV RNA. The results suggest that anti-N14E plus #19-#40 as well as HCV EIA II seems to be useful for the detection of patients with the HCV infection and that an improvement for the diagnosis of type C hepatitis and a decrease in the incidence of type C post-transfusion hepatitis can be achieved by using anti-N14E plus #19-#40.