The origin of myofibroblasts and the factors promoting their differentiation during liver fibrogenesis remain uncertain. During biliary-type fibrogenesis, the proliferation and chemoattraction of hepatic stellate cells (HSC) toward bile ducts is mediated by platelet-derived growth factor (PDGF), while myofibroblastic conversion of peribiliary cells distinct from HSC also occurs. We herein examined the phenotype of these peribiliary myofibroblasts as compared with myofibroblastic HSC and tested whether their differentiation was affected by PDGF. Biliary-type liver fibrogenesis was induced by common bile duct ligation in rats. After 48 hours, periductular fibrosis in portal tracts colocalized with smooth muscle alpha-actin-immunoreactive myofibroblasts, the majority of which were desmin negative. Simultaneously, in sinusoids, desmin immunoreactivity was induced in a large number of HSC, which were smooth muscle alpha-actin negative. Cultures of peribiliary myofibroblasts were expanded from isolated bile duct segments and compared with myofibroblastic HSC. Peribiliary myofibroblasts outgrowing from bile duct segments expressed smooth muscle alpha-actin, alpha1 (I) collagen mRNA, and PDGF receptor-beta subunit. Desmin immunoreactivity gradually decreased in cultured peribiliary myofibroblasts, contrasting with constant labeling of all myofibroblastic HSC. In addition, IL-6 expression in peribiliary myofibroblasts was up to 100-fold lower than in myofibroblastic HSC, whereas the expression of the complement-activating protease P100 in both cell types showed little difference and that of the extracellular matrix component fibulin 2 was similar. The expression of smooth muscle alpha-actin protein in cultured peribiliary myofibroblasts was stimulated by PDGF-BB and inhibited by STI571, a PDGF receptor tyrosine kinase inhibitor, whereas in bile duct-ligated rats, the administration of STI571 caused a significant decrease in peribiliary smooth muscle alpha-actin immunoreactivity, and to a lesser extent, a decrease in peribiliary fibrosis. These results indicate that peribiliary cells distinct from HSC undergo a PDGF-mediated conversion into myofibroblasts expressing IL-6 at lower levels than myofibroblastic HSC and contribute to the initial formation of biliary-type liver fibrosis.