Cytokeratins are the largest subfamily of intermediate filament proteins and include more than 20 different gene products, which are expressed in an epithelial tissue-specific manner. We studied by immunohistochemistry and confocal microscopy the distribution of cytokeratin subtypes in the biliary system of adult rat liver. A cytokeratin (CK)19+/7− cholangiocyte population was observed in the smaller branches of the biliary tree including the canals of Hering. They proliferated after 2-acetaminofluorene (AAF) administration, although later the typical oval cells expressed CK7. This observation suggests that cholangiocytes with this cytokeratin phenotype may harbor adult hepatic stem cells. The CK19+/7− cholangiocytes were not present in the rat liver at birth, but developed postnatally. Similar cell populations were not observed in human livers. In conclusion, we propose that the CK19+/7− phenotype may be characteristic for adult hepatic stem cells in rat liver and that these cells are generated de novo after birth. Supplementary material for this article can be found on the HEPATOLOGY website ( (HEPATOLOGY 2005;42:863–870.)