To obtain information on the mechanisms of hepatocellular damage and the determinants of response to interferon, hepatitis C virus (HCV) genotype, tissue HCV antigens, hepatocellular expression of HLA-A,B,C and intercellular adhesion-1 molecules, and the number of lobular T lymphocytes were studied in 38 anti-HCV-positive patients. 14 patients did not show a primary response to interferon treatment. HCV genotype 1b was detected in 11 of them. They displayed higher scores of HCV-positive hepatocytes, HLA-A,B,C, and ICAM-1 molecules expression than with the responders. HCV-infected hepatocytes maintained the capacity to express HLA-A,B,C and ICAM-1 molecules. CD8-positive T cells in contact with infected hepatocytes and Councilman-like bodies were observed. A significant correlation was found between the number of lobular CD8-positive T cells and alanine amino transferase levels. No differences were observed in clinical, biochemical, and histological features between patients with high and low number of hepatocytes containing HCV antigens. These data suggest a prominent role of T cell-mediated cytotoxicity in the genesis of hepatocellular damage. The high expression of interferon-inducible antigens like HLA-A,B,C molecules suggests the presence of strong activation of the interferon system possibly related to high HCV replication in nonresponder patients infected with genotype 1b.