The platelet-derived growth factor (PDGF) binds specifically to high-affinity receptors on the surface of bovine aortic smooth muscle cells and 3T3 cells. Conditioned medium from cultured bovine aortic endothelial cells (EC) prevents PDGF binding to these receptors in a dose-dependent manner at 4 degrees C. The (125)I-labeled PDGF that is displaced by the conditioned medium shows no increase in trichloroacetic acid solubility or decrease in binding capability to fresh cells. The competitor activity was identified as a protein by ammonium sulfate precipitability and sensitivity to trypsin. The competitor protein also is found in the serum-free conditioned media from porcine aortic EC and human umbilical vein EC but not in media from bovine aortic smooth muscle cells, human neonatal foreskin fibroblasts, or the interleukin-producing thyoma cell line EL-4. The competitor protein, like PDGF, has no effect on the specific 4 degrees C binding of either (125)I-labeled insulin to 3T3 cells or (125)I-labeled epidermal growth factor to human epidermoid A431 cells. Saturation curves of PDGF binding to smooth muscle cells that had been preincubated in the presence and absence of competitor indicate that the concentration for half-maximal binding of (125)I-labeled PDGF to its receptor ( approximately 30 pM) is unchanged by the competitor, whereas the apparent number of available receptor sites or maximal level of binding is greatly diminished. The competitor activity produced by cultured human umbilical vein EC is completely inhibited by antiserum against pure human PDGF, whereas the same PDGF antiserum only partially inhibits the mitogenic activity of the conditioned media. In addition, approximately 7-fold more crude endothelium-derived growth factor is required for half-maximal inhibition of (125)I-labeled PDGF binding as is required for half-maximal stimulation of DNA synthesis. These results suggest that EC secrete a PDGF-like protein that is biochemically distinct from the majority of EC-derived mitogenic activity.