Abstract These studies investigate the role of transforming growth factor-β1, a potent inhibitor of epithelial cell proliferation and stimulator of extracellular matrix biosynthesis, during intrahepatic bile duct hyperplasia and biliary fibrosis. These pathogenic responses were induced in rats by common bile duct ligation. Bile duct cell replication, measured by the bromodeoxyuridine labeling index, was significantly increased 24 hr after common bile duct ligation. This response diminished to baseline by 1 wk. Liver collagen content, determined by quantification of hydroxyproline, was increased significantly after 1 wk of common bile duct ligation, and by 4 wk was increased by a factor of 4. Immunohistochemistry revealed low levels of TGF-β1 in normal intrahepatic bile duct epithelium. In contrast, the bile duct epithelium in bile duct-ligated rats stained strongly positive for transforming growth factor-gb1 at 1 and 4 wk after ligation. These results suggest that transforming growth factor-β1 may play a role in both the termination of the bile duct epithelial cell proliferative response and the induction of fibrogenesis after common bile duct ligation. In addition, the mannose 6-phosphate/insulin-like growth factor II receptor was up-regulated in hyperplastic bile duct epithelium 1 and 4 wk after ligation. Because the mannose 6-phosphate/insulin-like growth factor-II receptor has been shown to facilitate the proteolytic activation of transforming growth factor-β1, these results suggest that the bile duct epithelium may also be involved in the activation of transforming growth factor-β1.