Hepatitis C virus has been recognized as a major cause of non-A, non-B viral hepatitis. Although serologic tests have been commercialized, no specific histological or immuno-histochemical markers for hepatitis C virus infection are available for routine use. In an effort to detect hepatitis C virus antigen in liver tissue we investigated the immuno-reactivity to monoclonal antibodies on frozen liver tissue from a chimpanzee and patients with chronic non A, non B hepatitis. Monoclonal antibodies were developed in mice immunized with a synthetic peptide derived from hepatitis C virus core antigen. One monoclonal antibody was reactive and showed typical cytoplasmic granules in chimpanzee hepatocytes. Using this monoclonal antibody a similar staining pattern was found in the liver biopsies of 21 out of 28 chronic non-A, non-B hepatitis patients, positive for hepatitis C virus-RNA and anti-HCV. The granular immuno-reactivity was abolished after pre-incubation of this monoclonal antibody with infected chimpanzee liver or with hepatitis C virus synthetic peptide but not with normal chimpanzee or human liver tissue. There was no reactivity in four patients with hepatitis C virus-RNA-negative, anti-HCV-positive chronic non-A, non-B hepatitis, in 11 patients with chronic type B hepatitis or in 12 hepatitis C virus-RNA-negative, anti-HCV-negative patients with various liver diseases. However, staining was found in three out of four additional chronic type B hepatitis patients suspected of co-infection with non-A, non-B agents.(ABSTRACT TRUNCATED AT 250 WORDS)