The embryonal origin of hepatic stellate cells (HSCs), the principal cells in hepatic fibrogenesis, is still intriguing. To explore the origin and the differentiation of HSCs, we studied the expression of cytokeratin 18 (CK18) and 19 (CK19), the standard markers of simple epithelial cells, in cultured human HSCs. Hepatic stellate cells were isolated from five normal human livers. In immunofluorescence staining, both clone C-51 anti-CK18 antibody and clone RCK108 anti-CK19 antibody labeled almost all stellate cells in the primary culture. Double immunofluorescence staining for cytokeratin/vimentin and cytokeratin/alpha-smooth muscle actin detected by confocal laser scanning microscopy clearly demonstrated the localization of cytokeratin immunoreactivity in human HSCs. During subsequent cultivation of human HSCs to the tenth passage, immunocytochemical staining and western blot analysis demonstrated gradually decreasing profiles of CK18 and CK19 expression. The progressive reduction of cytokeratin expression was further confirmed in a culture of clone cells originated from a single HSC. In conclusion, both CK18 and CK19 are expressed in cultured human HSCs, and the extent of their expression decreases gradually during prolonged cultivation. Our results suggest that HSCs may be of epithelial origin, and that they undergo the transdifferentiation from epithelial to mesenchymal phenotype during an activation process in vitro.