MUC1 apomucin is a specific target tumor antigen recognized by cytotoxic T cells in a major histocompatibility complex (MHC) unrestricted fashion in patients with pancreatic and breast cancer. This T-cell-mediated immune mechanism against MUC1 apomucin expressing cells has not been evaluated in nonneoplastic immune-mediated diseases. Therefore, we immunohistochemically surveyed the expression of MUC1 apomucin on biliary epithelial cells of small bile ducts in various hepatobiliary diseases, including primary biliary cirrhosis (PBC). MUC1 apomucin was detected using the monoclonal antibody DF3 and the streptavidin-biotin complex, in livers from 31 patients with PBC, 67 with chronic viral hepatitis (CH) with or without cirrhosis, 31 with extrahepatic biliary obstruction (EBO), 30 with hepatolithiasis, and from 23 normal individuals. MUC1 apomucin was infrequently and focally expressed in the biliary epithelial cells of the small bile ducts in 3 of 23 normal livers. In contrast, MUC1 apomucin was frequently and strongly expressed on the luminal surface of biliary epithelia] cells of small bile duct, in 22 of 31 patients with PBC, and in 50 of 67 patients with CH. In particular, high levels of MUC1 apomucin were expressed in bile ducts showing chronic nonsuppurative destructive cholangitis (CNSDC) in PBC and hepatitic duct injuries in CH. In EBO and hepatolithiasis, MUC1 apomucin was focally and weakly expressed in 29% and 30% of livers examined, respectively. More MUC1 apomucin was expressed in PBC and CH than in EBO, hepatolithiasis, and normal liver (P < .01, respectively). Frequent and high luminal expression of MUC1 apomucin on biliary epithelial cells in damaged small bile ducts in PBC and CH may be related to T-cell-mediated immunologic mechanisms in these diseases, probably by an MHC-unrestricted recognition process.