A series of 10 cases of biliary obstruction due to primary cholangiocarcinoma has been studied with histological and immunocytochemical means. The total duration of cholestasis (as manifested by jaundice) was between 2 and 11 weeks with variable period of preoperative drainage. Liver biopsy specimens taken during surgery for cholangiocarcinoma were investigated for the presence of ductular proliferation and the development of fibrosis, as demonstrated by Sirius Red F3BA collagen staining. The differentiation of epithelial components was evaluated by AEC-immunostaining with chain-specific monoclonal antibodies specifically directed against human keratins type 7, 18 and 19. Keratin 7, normally occurring only in the ductular system, was expressed in hepatocytes at the periphery of the hepatic lobule (zone I) following about 4 weeks' cholestasis, when an increase of ductular profiles in the enlarged portal areas had become manifest. Such keratin 7 positive cells, however, still retained all morphological aspects of hepatocytes. Keratin 19, normally also restricted to the ductular system in liver, is not expressed by zone I hepatocytes even after longer duration (up to 11 weeks) of cholestasis. It is concluded that the increase in ductular profiles during the first week is mainly due do proliferation of pre-existing ductules, while ductular metaplasia occurs in more chronic cholestasis. Development of fibrosis, not always strictly paralleling the multiplication of ductular profiles in sections through a portal tract, represents an early change, and is clearly apparent after 2 weeks of obstruction.