Ultrastructural and immunohistochemical study was conducted on the intrahepatic peribiliary capillary plexus in normal livers and in those with extrahepatic biliary obstruction. In both conditions, capillaries positive for Ulex europaeus agglutinin I and type IV collagen were always present in the vicinity of the bile ducts. Immunoelectron microscopy showed the presence of type IV collagen on the basal lamina of these capillaries; Ulex europaeus agglutinin I was also positive on their cytoplasms. Under electron microscope, a considerable number of these capillaries were seen as being composed of fenestrated endothelium with a diaphragm and with extreme cytoplasmic attenuations that were dense at the sides facing the bile duct in comparison with the opposite sides in normal livers. In extrahepatic biliary obstruction, plasmalemmal pinocytic vesicles, multivesicular bodies and other cellular organellae such as rough endoplasmic reticulum and Weibel-Palade bodies increased in number in these capillaries, relative to normal livers, probably reflecting increased permeability and functional activities. These characteristic ultrastructural features of the peribiliary capillary plexus might be arranged to transport substances effectively by way of intrahepatic biliary epithelial cells in normal livers and also might be altered to meet the increased functional demands of extrahepatic biliary obstruction. (Hepatology 1992;15:411–418).