Expression of the platelet-derived growth factor receptor (PDGFR) was detected by immunocytochemistry in normal and malignant mesothelial cells from 14 benign effusions and 22 mesotheliomas. Two well-characterized antireceptor monoclonal antibodies to the PDGFR alpha-subunit (PR292) and beta-subunit (PR7212) were used. PDGFR alpha-subunit outlined cell membranes intensely in nonneoplastic mesothelial cells, whereas it was focal in mesothelioma and limited to a few cases only. PDGFR beta-subunit was weakly expressed in the cytoplasm of normal mesothelium; in contrast, malignant mesothelial cells showed strong cytoplasmic staining. Our results show that normal mesothelium may be responsive to PDGF by the predominant expression of PDGFR-alpha and less by PDGFR-beta and indicate the presence of growth-stimulation loops in mesothelioma through PDGF/PDGFR-beta interaction. Also, scattered staining for the PDGFR alpha-subunit suggests that a PDGF-A chain/PDGFR-alpha interaction may also be active in mesothelioma cells growing in suspension.