Both liver epithelial and oval cells are believed to be liver stem cells. We investigated the identification by producing monoclonal antibodies against liver epithelial cells. Monoclonal antibodies against hepatic stem-like cells (HSL cells) have been selected to follow the hepatic stem cells during hepatic regeneration and developmental changes in the liver. Monoclonal antibodies were induced by immunization of BALB/c mice with HSL cells established from the epithelial cells of the adult rat liver. The hybridomas were screened by indirect immunofluorescence staining of HSL cells. We produced a unique monoclonal antibody against HSL cells, MabH, which specifically recognizes liver epithelial cells. MabH did not react with liver parenchymal cells but did react with bile ductule cells under normal conditions in the adult liver. This antibody also reacted with oval cell lines and with the oval cells that appeared during liver regeneration. In addition, fetal liver cells showed immunoreactivity with MabH. Although the level of staining decreased after birth, some cells in the portal area remained highly reactive. These results suggested that liver epithelial cells, oval cells, and fetal liver cells possess a common cell marker of liver stem cells.