Connective tissue components were detected immunohistochemically in cultured rat hepatocytes and sinusoidal lining cells. In the Ito and endothelial cells, the staining reactions to prolyl hydroxylase were strong. The reaction in the hepatocytes, however, was weaker, and there was no staining reaction to prolyl hydroxylase in the Kupffer cells. In the Ito cells, types III and IV collagen were clearly positive, whereas reactions to type I collagen were very weak. In the endothelial cells, type IV collagen stained clearly, whereas the reaction for type I collagen was very weak, and there was no reaction for III. Laminin was clearly positive in the Ito and endothelial cells. Connective tissue components were not detectable in the Kupffer cells. The hepatocytes were positive for types I, III, and IV collagen, but the staining reactions to type IV collagen were weak. Laminin staining was almost negative. These results suggest that type IV collagen is produced mainly by nonparenchymal cells and that types I and III collagen are produced by both parenchymal and nonparenchymal liver cells.