We have attempted to determine the cell types producing laminin in normal and fibrotic human liver by in situ hybridization to laminin B1 chain gene transcripts with [35S]-labeled RNA probes. In normal liver laminin transcripts were detected in vascular endothelial, in bile duct epithelial, and in portal tract mesenchymal cells as well as in perisinusoidal cells of the lobule. In fibrotic liver, expression of laminin RNA was increased in proliferating bile duct epithelial cells, stromal cells of fibrotic septa, and in perisinusoidal cells of the regenerating nodules. Although recently suggested as a major source of laminin, hepatocytes in none of our samples showed evidence for significant laminin B1 gene transcription. Thus laminin synthesis does not appear to be a function of hepatocytes, but rather of mesenchymal, endothelial, and bile duct epithelial cells in the liver. This pattern of laminin expression underlines the importance of the mesenchyme in conditioning the function and differentiation of the hepatic parenchymal compartment.