The cell phenotype of so-called bile duct adenoma (BDA) was investigated immunohistochemically using monoclonal antibodies to two recently identified antigens (designated D10 and 1F6) extracted from human liver and cultured biliary epithelium. The acini and tubules of BDA consisted of serous and mucous cells that expressed D10 and 1F6. The intrahepatic peribiliary glands of normal liver, comprising intramural mucous glands and extramural tubuloalveolar seromucinous glands, similarly expressed D10 and 1F6 antigens. Antigen 1F6 was present in the cells forming the canals of Hering and normal bile ductules but not in interlobular and larger bile ducts. Proliferating bile ductules associated with large bile duct obstruction and alcoholic cirrhosis or the epithelia of the von Meyenberg complex and polycystic liver did not exhibit this combined profile of D10 and 1F6 expression and mucous cells. These findings suggest an origin of BDA from peribiliary glands rather than from bile ductules or ducts. Consistent with this view was our finding that 18 of the 30 BDA were spatially related to a large-calibre bile duct. Therefore, BDA, well known for its benign behavior is a small mass of disorganized but mature peribiliary gland acini and tubules within a variable amount of stroma and should properly be called a peribiliary gland hamartoma.