Human arterial smooth muscle cells (hASMC) were cultured from explants of the inner media of uterine arteries obtained at hysterectomy. The presence of alpha-actin and smooth muscle-specific actin isoforms and the microscopic appearance of the cells in secondary culture established their smooth muscle origin. The hASMC were diploid and had no signs of transformation. Plasma-derived serum failed to stimulate their proliferation in vitro. Their rate of proliferation was, however, proportional to the concentration of whole blood serum in the medium. Anti-PDGF IgG at high concentrations inhibited the stimulatory effect of whole blood serum on cell proliferation. This suggests that hASMC depend on exogenous PDGF for their growth. In PDS or bovine serum albumin cell numbers remained constant for 7 days in culture and the thymidine index was below 1% per 24 h. When reexposed to whole blood serum these cells started to proliferate within 2 days. This indicates that hASMC when deprived of PDGF enter a quiescent state that is fully reversible upon rexposure to the mitogen. Heparin is a powerful growth inhibitor for SMC. In our system, heparin caused a dose-dependent inhibition of cell proliferation despite optimal concentrations of whole blood serum. This inhibition was reversible upon withdrawal of heparin. At heparin concentrations which caused a half-maximal inhibition it was also competed for by increasing concentrations of whole blood serum. Quiescent hASMC expressed the PDGF receptor on their surface as judged from immunofluorescence with a monoclonal antibody. This was true irrespective of whether growth arrest was achieved by serum depletion or by the addition of heparin to serum-containing medium. Cells growing in the presence of whole blood serum did not, however, express the receptor antigen. These observations suggest that heparin may interfere with PDGF or with its binding and further processing at the level of the cell-surface receptor.