The distribution of blood group-related antigens (A, B, H, Leb) in the intrahepatic biliary tree and hepatocytes was studied by immunoperoxidase techniques. In normal livers, large and septal bile ducts expressed A and B antigens in the cases with comparable blood groups and also expressed H antigen frequently in the cases with blood group O, A or B, and infrequently in the cases with AB. Lea and Leb were found in biliary cells at any level in secretors. In hepatobiliary diseases, A, B and H were neoexpressed in the interlobular bile ducts in addition to their expression in the large and septal bile ducts. Lea and Leb were found in almost all proliferated bile ductules and in some hepatocytes, especially those adjacent to ductules, in any of the hepatobiliary diseases, and their expression seemed parallel to the disease activity. H antigen was also neoexpressed in some hepatocytes and proliferated bile ductules. Extramural peribiliary glands expressed H, Lea, Leb and, to a lesser degree, A and B antigens in normal and pathologic conditions. This study disclosed that the normal biliary tree has a specific expression of blood group antigens at different levels and that this expression is altered in pathologic conditions. Periportal hepatocytes showed neoexpression of blood group antigens in pathologic conditions, and some of these hepatocytes may undergo ductular metaplasia.