Anti-hepatitis C virus (HCV) core antibodies in blood donors were detected in enzyme-linked immunosorbent assay with oligopeptides (CP9 and CP10) corresponding to the putative core protein of HCV and their performances were compared with C100-3 antibodies. HCV-RNA was detected by nested polymerase chain reaction in fresh plasma samples from blood donors positive for anti-core antibodies in hihg-titers and/or anti-C100-3 antibodies. In plasma samples from 2, 034 apparently healthy blood donors, anti-CP9 was detected in 133 (6.5%), anti-CP10 was detected in 95 donors (4.7%) and anti-C100-3 was detected in 33 donors (1.6%). Among plasmas positive by at least one test, 31 were found to contain HCV-RNA; 19 of them were negative for anti-C100-3, but positive for anti-CP9 and/or anti-CP10 in high titers (OD≥1.000). These results indicated that anti-core antibodies would be useful in complementing anti-C100-3 for further decreasing the incidence of post-transfusion non-A, non-B hepatitis.