AbstractSerial serum samples from 35 patients with posttransfusion non‐A, non‐B hepatitis in a prospective study were tested for antibody to hepatitis C virus (HCV) by a multiple recombinant antigen based immunoassay [anti‐HCV (2nd); Abbott], Of them, 23 were positive for anti‐C100, and 27 were positive for HCV RNA by polymerase chain reaction. By anti‐HCV (2nd), 28 patients were positive, and all except 1 of the 28 patients were positive for HCV RNA. A total of 24 patients, who became HCV RNA positive at the acute stage and who were negative for both anti‐C100 and anti‐HCV (2nd) before transfusion, were considered to have posttransfusion hepatitis C. The mean time to seroconversion for anti‐HCV (2nd) was 7.5 or 12.1 weeks after the onset of hepatitis or the date of transfusion, respectively, and was generally 6 weeks earlier than that detected by anti‐C100. However, seroconversion was delayed in 2 hepatitis B surface antigen carriers as compared with anti‐C100. The anti‐C100 assay detected 20 (83%) and the anti‐HCV (2nd) all 24 patients with documented posttransfusion hepatitis C. The second‐generation test is, therefore, better than conventional anti‐C100 for the early