The expression of Lewis blood group antigens, Lewisa, Lewisb, Lewisx and Lewisy, in 40 non-cancerous livers and in 20 hepatocellular carcinomas (HCC), 5 cholangiocarcinomas (CC), and 6 combined hepatocellular and cholangiocarcinomas (Comb) was studied by immunohistochemistry using monoclonal antibodies. Although normal hepatocytes did not express any of the four Lewis antigens, Lewisy expression was detected in some hepatocytes at the periphery of pseudolobules in cirrhotic liver. While Lewisx in bile duct epithelial cells was undetectable in normal livers, it was detected in some cases with non-cancerous liver diseases. Lewisx and Lewisy were detected in 30% of HCC and 100% of CC and Comb. Non-cancerous bile duct epithelial cells in almost all instances were stained strongly by anti-Lewisa and/or anti-Lewisb antibody. Proliferating bile ductules within Glisson's sheath and pseudolobules could be clearly detected by Lewisa and Lewisb staining. Lewisa and Lewisb were never detected in non-cancerous hepatocytes, and were only rarely detected in HCC, but were expressed in most cases of CC and Comb. These results suggested that Lewisa and Lewisb are useful markers for differentiation towards biliary epithelial cells in the liver, and that Lewisx and Lewisy expression might be associated with states of increased or altered cell proliferation.