We have investigated the effects of interactions between growth factors and heparin-like glycosaminoglycans on untransformed human arterial smooth muscle cells (hASMC) in vitro. The results indicate that heparin in the presence of serum mitogens prevents the cells from entering the S phase of the cell cycle by binding and inactivating reversibly some serum mitogen(s). Our results suggest that platelet-derived growth factor (PDGF) is one of them and that it is the most potent stimulator of hASMC growth in vitro. Thymidine incorporation as well as increase in DNA content was inhibited not only by the presence of heparin in serum-containing medium but also when serum was chromatographed on Heparin-Sepharose at physiologic salt concentrations before exposure to the cells. The mitogenic activity of the unretained serum fraction was restored by the addition of PDGF AA, AB, or BB dimers or of a fraction (RF I) that dissociated from Heparin-Sepharose at 0.2 to 0.6 M NaCl. Radiolabeled recombinant PDGF (c-sis) dissociated from Heparin-Sepharose within a concentration range of NaCl similar to that of RF I. Neither the unretained material nor the RF I or PDGF dimers were effective alone. The effect of RF I was significantly decreased by the addition of an anti-PDGF IgG that is known to neutralize the PDGF mitogenic activity partially. Addition of heparin abolished DNA-synthesis when the PDGF dimers or RF I were combined with the unretained fraction. A second fraction (RF II) bound strongly to Heparin-Sepharose and eluted between 1.1 and 1.6 M NaCl. The RF II also induced DNA synthesis but was neither as efficient as RF I nor depending on other serum fractions for growth promotion and it was not inhibited by anti-PDGF IgG. A similar strong affinity for Heparin-Sepharose was found for labeled basic fibroblast growth factor and we cannot exclude the possibility that RF II represent fibroblast growth factor. Under these culture conditions, inhibition of hASMC proliferation was directly correlated with the expression of smooth muscle specific alpha actin isoforms in stress fibers and the suppression of a proliferating cell-specific nuclear antigen. Conversely, stimulation of hASMC proliferation was associated with the opposite phenomenon. We conclude that heparin-like glycosaminoglycans influence growth and phenotype of hASMCs in vitro by binding and inactivating PDGF. Inasmuch as heparin-like substances constitute a significant proportion of the proteoglycan-associated glycosaminoglycans of the arterial wall, such mechanisms might be important for the development of atherosclerotic lesions.