ABSTRACT— Surgically obtained liver specimens from four patients with primary biliary cirrhosis (PBC), Stages I–II, were studied immunohistochemically using a broad panel of monoclonal antibodies. At the site of bile-duct injury (CNSDC), the inflammatory cells were recognized to be Coulter T-11+ (directed against all T-cells) and OKT-8+ (directed against cytotoxic/suppressor: C/S T-cells) cells. The MHC-class I antigen (i.e. HLA-A, B, C) was expressed weakly in the cytoplasm of the minority of damaged bile-duct epithelial cells, and the MHC-class II antigen (i.e. HLA-DR) was not expressed. Thus, OKT-8+ cells may play an important role in the immunologically mediated destruction of ductular epithelium in PBC. Strong MHC-antigen expression and OKT-8+ cell infiltration in destructive bile-duct lesions were not simultaneously observed. In the portal lymphocyte-rich areas, OKT-4+ (directed against helper/inducer: H/I T-cells) predominated over OKT-8+ cells. B-lymphocytes were present predominantly in the peripheral zones of the lymphoid aggregates. The distribution of helper/inducer T-cells and B-lymphocytes indicates that they may play a role in the induction of immunoglobulin in the lymphocyte-rich areas. The inflammatory mononuclear cells within the granuloma observed in one patient were Coulter T-11+, OKT-8+ and Leu-3a+3b+ (directed against helper/inducer T-cells) cells.