Current ideas about the mechanism of wound healing and the pathogenesis of atherosclerosis, pulmonary fibrosis and hepatic fibrosis suggest a central role for the mononuclear phagocyte in attracting and/or stimulating the proliferation of mesenchymal cells^1–5. We demonstrate here that activated human blood monocytes, but not resting monocytes, release a mediator that attracts smooth muscle cells and cooperates with other mediators to stimulate fibroblast proliferation. This mediator is very similar to platelet-derived growth factor (PDGF): its Chromatographic properties and chemical stability are similar to those of PDGF^6,7, it competes with ¹²⁵I-PDGF for binding to fibroblasts and it immunoprecipitates with anti-PDGF antibodies. In parallel, stimulated monocytes, but not resting monocytes, express the c-sis proto-oncogene, a gene coding for one of the PDGF chains^8,9, consistent with the concept that expression of the c-sis proto-oncogene may be involved in the ability of mononuclear phagocytes to modulate the accumulation of mesenchymal cells.