Expression of ABH blood group antigens, Ulex europaeus agglutinin I, and type IV collagen in the sinusoids of hepatocellular carcinoma. The expression of blood group antigens (A, B, H, Lewis(a) and Lewis(b)), Ulex europaeus agglutinin I (UEA-I), factor VIII-related antigen, and type IV collagen on the sinusoids was examined immunohistochemically in 15 cases of hepatocellular carcinomas (HCC), 11 cases of cirrhosis, 12 cases of chronic active hepatitis, and in a control sample of 16 normal livers. Sinusoidal endothelial cells of HCC characteristically showed a diffuse and strong immunoreactivity to ABH blood group antigens in the specimen with a comparable ABO blood group. The sinusoidal endothelial cells were also diffusely and strongly positive for UEA-I receptors. In contrast, in cirrhosis and chronic active hepatitis a few sinusoidal endothelial cells were positive for ABH blood group antigens and UEA-I receptors. In normal livers, only a few sinusoidal endothelial cells were positive for ABH blood group antigens and UEA-1 receptors. Tests for factor VIII-related antigen and Lewis blood group antigens were almost negative on sinusoidal endothelial cells. Although type IV collagen was distributed diffusely in the space of Disse in these four groups, its expression was strongest in HCC. Blood vessels of portal tracts and fibrous septa were positive for ABH blood group antigens, UEA-1 receptors, factor VIII-related antigen, and type IV collagen, but negative for Lewis blood group antigens. These findings suggest that some sinusoidal endothelial cells undergo "capillarization" in cirrhosis and chronic active hepatitis, and that the majority of sinusoidal endothelial cells of HCC have phenotypic characteristics of capillaries. 