The results of an immunohistochemical study of platelet-derived growth factor receptor (PDGF-R) beta chain in human non-neoplastic mesothelium (35 cases) and in malignant mesothelioma (33 cases) using a murine monoclonal antibody are reported. In approximately half of the malignant mesotheliomas, there was immunoreactivity in the cytoplasm of the neoplastic cells for the beta chain of the PDGF-R. In some cases, there was also membrane immunoreactivity. There were no statistically significant differences between the various mesothelioma subtypes. Immunoreactivity for PDGF-R beta chain was absent in non-neoplastic mesothelium.