Rat alveolar macrophages secrete a growth factor that renders rat lung fibroblasts competent to initiate DNA synthesis in vitro in the presence of platelet‐poor plasma. This biological activity resembles that of platelet‐derived growth factor (PDGF). After separation from putative associated binding proteins by chromatography under acidic conditions, the macrophage‐derived factor exhibited a relative molecular weight similar to that of highly purified human PDGF. The factor bound to a monospecific antibody to human PDGF and thus could be quantitated in an enzyme immunoassay for PDGF. It competed with radiolabeled human PDGF for receptor sites for PDGF on rat lung fibroblasts, and binding to these receptor sites could be specifically inhibited by anti‐PDGF. These data strongly support the view that the factor derived from rat alveolar macrophages is homologous to human PDGF and is similar to human macrophage‐derived PDGF‐like growth factor. Furthermore, we have demonstrated that the lung contains both an effector cell (pulmonary macrophage) and a potential target cell (interstitial fibroblast) for this cytokine. Therefore the rat appears to be an appropriate animal model in which to study macrophage‐derived PDGF‐like growth factors as mediators of proliferation in pulmonary fibrogenesis.— Kumar, R. K.; Bennett, R. A.; Brody, A. R. A homologue of platelet‐derived growth factor produced by rat alveolar macrophages.FASEB J.2: 2272‐2277; 1988.