We analyzed the immunophenotypes of hepatic cellular infiltrates by quantitative immunohistochemical methods in biopsy specimens from 20 patients with primary biliary cirrhosis (PBC), 19 with primary sclerosing cholangitis, and 11 with autoimmune chronic active hepatitis. Specifically, we sought to identify activated T cells, interferon-gamma-producing cells, and natural killer cells. The portal cellular infiltrate in PBC contained a preponderance of CD4 cells in comparison with CD8 cells, with a CD4/CD8 ratio of 2.45:1. The cellular infiltrate in areas of piecemeal necrosis contained mostly CD8 cells. Infiltrating CD8 cells in PBC had the surface phenotype of cytotoxic (CD8-positive, CD11b-negative) cells. Approximately 4% of T cells expressed interleukin 2 receptors. Interferon-gamma-staining cells were rarely identified (in less than 2%). Cells that expressed the natural killer cell markers CD16, CD56, or CD57 were infrequent, constituting approximately 5% of the cellular infiltrate. The composition of the infiltrates was similar in patients with PBC and chronic active hepatitis. Natural killer cells were twice as common in patients with primary sclerosing cholangitis (P < 0.05) as in those with PBC. The inflammatory infiltrates in areas of piecemeal necrosis were similar in the three diseases and differed from those found within the portal area, in that CD8 cells were preponderant. In all three liver diseases, almost 90% of bile ducts expressed class II HLA antigens. These findings support the hypothesis that cytotoxic T cells of either the CD4 or CD8 immunophenotype but not natural killer cells may be involved in the pathogenesis of PBC and chronic active hepatitis.(ABSTRACT TRUNCATED AT 250 WORDS)