Ten cases of malignant mesothelioma (MM), as diagnosed by clinical history and light and electron microscopy, were studied with polyclonal antibodies directed against the basement membrane-specific proteins, type IV collagen and laminin, as well as with monoclonal antibodies which recognize two epitopes of the laminin receptor (LR). All formalin-fixed, paraffin-embedded mesothelioma tissues examined demonstrated intracytoplasmic immunoreactivity for the basement membrane proteins. Extracellular staining was minimal, analogous to the staining reactions observed in adenocarcinomas of the breast and lung, which on light microscopy mimicked the morphologic appearance of MM. Similarly, LRs were identified on MM cells by intense positive staining. Immunoreactivity was also evident on nonneoplastic mesothelioma and adenocarcinoma cells but with greater heterogeneity and less intensity. It may be concluded from these results that (a) malignant mesotheliomas have the ability to synthesize components of the basement membrane; (b) enhanced attachment to extracellular matrix by MM would be anticipated as laminin receptors are present in large numbers on the surface of mesothelioma cells; (c) the reason for lack of intensiveness by MM cells remains speculative; type IV-specific collagenase may play a role in regulating this function in these tumor cells.