We investigated the development of human intrahepatic peribiliary capillary plexus by lectin-histochemistry and immunohistochemistry, using 40 fetal livers of various gestational ages, 52 postnatal livers of various ages, and 10 adult livers. In the earliest developmental stage of intrahepatic bile ducts ("the stage of ductal plate"), no vasculatures were found around the ductal plate, but progenitor vascular cells positive for von Willebrand factor, Ulex europaeus agglutinin I and succinylated wheat germ agglutinin appeared in the mesenchyme of the portal tracts. In "the stage of biliary cell migration into the mesenchyme," the progenitor vascular cells transformed into capillaries positive for von Willebrand factor, Ulex europaeus agglutinin I, and succinylated wheat germ agglutinin. In "the stage of bile duct formation," capillaries began to surround the bile ducts (immature peribiliary capillary plexus). The capillaries of the immature peribiliary capillary plexus proliferated and formed premature peribiliary capillary plexus around 40 wk of gestation in large bile ducts and around 6 wk after birth in small bile ducts. The former was composed of inner and outer layers, whereas the latter consisted of scattered capillaries without layer formation. The premature peribiliary capillary plexus continued to proliferate in the postnatal life and reached an adult and mature state around 15 yr. These data suggest that endothelial cells of capillaries of peribiliary capillary plexus derive from mesenchyme at the earliest stage of bile duct development and that the development and maturation of peribiliary capillary plexus progress parallel to those of the intrahepatic bile ducts. (HEPATOLOGY 1993;18:529-536.) The intrahepatic bile ducts are supplied and nourished by a network of capillaries called peribiliary capillary plexus (PBP) (1-8), and the anatomical relationship between the intrahepatic bile ducts and PBP is important in maintaining the normal function of intrahepatic bile ducts (7). The previous studies, mainly using animals, disclosed that PBP is composed of smaller