Phenotypic alterations of biliary epithelial cells such as changes in intermediate filament composition and presence of carbohydrate residues, occur during the development of intrahepatic bile ducts. In this study, we examined the expression of MUC1, MUC2, MUC3, and MUC5/6 apomucins (mucin core proteins) by immunohistochemical means in the human intrahepatic bile duct during its development and maturation. In the fetal liver, new bile ducts in the portal tracts, either at the hilar level (corresponding to the large bile ducts) or peripheral level (corresponding to the small bile ducts), frequently expressed MUC1 apomucin at their luminal surface. Ductal plates also focally expressed MUC1 apomucin. By contrast, in the postnatal liver, the biliary epithelial cells of intrahepatic large bile ducts constantly expressed MUC3 apomucin, whereas those of small bile ducts did not. MUC2 and MUC5/6 apomucins were absent in the intrahepatic biliary elements of the fetal as well as postnatal livers. These data suggest that the biliary epithelial cells switch MUC1 apomucin expression before birth to that of MUC3 after birth. This characteristic transition may be similar to the changes in the hepatocellular expression of alpha-fetoprotein and albumin during the perinatal period.