A newly developed antibody assay based on a synthetic peptide of 15 amino acids derived from the core region of the hepatitis C virus (HCV) genome was evaluated in serum and plasma panels of (A) 225 haemophiliacs and (B) 44 patients with chronic non-A, non-B (NANB) hepatitis, and in (C) sequential serum samples of 9 patients with transfusion transmitted HCV infection. The new anti-core peptide ELISA was compared with the anti-C100 ELISA. For confirmation of HCV infection, samples were tested in a 4-antigen recombinant immunoblot assay (4-RIBA) and samples of panels B and C were also assayed in cDNA-polymerase chain reaction (PCR). In two panels with a high prevalence of HCV infection (88.4 and 70.5% in haemophilia and NANB hepatitis patients, respectively), the sensitivity of the anti-core peptide ELISA did not differ significantly from the sensitivity of the anti-C100 ELISA. The sensitivity of the new assay as compared with the anti-C100 assay was found to be 0.84 [95% confidence interval (CI): 0.78-0.89] versus 0.92 (95% CI: 0.87-0.95) in haemophilia patients and 0.71 (95% CI: 0.52-0.86) versus 0.84 (95% CI: 0.66-0.95) in NANB hepatitis patients. In sequential serum samples of patients with transfusion-transmitted HCV infection antibodies to the core peptide (in 6/9 patients) appeared later than antibodies to C100 (in 7/9 patients): 168 (range: 70-322) and 143 (range: 59-365) days after transfusion, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)