The localization of hepatitis C virus-infected hepatocytes in the human liver remains unclear despite the development of a serological assay for the antibody to hepatitis C virus. We studied their localization immunohistochemically with monoclonal antibodies to core, envelope and NS3 antigens of hepatitis C virus. We examined 48 liver biopsy samples from C100-3 antibody-positive patients with chronic liver disease (chronic persistent hepatitis, 5 cases; chronic active hepatitis, 41 cases; cirrhosis, 2 cases) and 12 liver biopsy samples from C100-3 antibody-negative patients with chronic liver disease (type B chronic hepatitis, 8 cases; alcoholic liver disease, 4 cases). In the C100-3 antibody-positive group, positive immunostaining for core antigen, envelope antigen and NS3 antigen was found in 23% (11 of 48), 24% (11 of 45) and 24% (11 of 46), respectively. Negative results were obtained in the C100-3 antibody-negative group. Hepatocytes with positive staining were scattered in the lobules, and they were found in the same regions irrespective of whether the antibody to core antigen, to envelope antigen or to NS3 antigen was used. Each positive cell was strongly stained in the cytoplasm; these decorations disappeared after absorption of the primary antibody with purified antigen. mean ALT levels in the patients with positive immunostaining for core, envelope or NS3 antigen (174.8 +/- 105.7 U/L) tended to be higher than in those with negative immunostaining (142.0 +/- 93.8 U/L). On histological evaluation of liver specimens with a scoring system of the histological activity index, intralobular inflammation and fibrosis had higher scores for samples with positive rather than negative immunostaining (p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)