We studied in vitro the effects of heparin on the growth of rat aortic smooth muscle cells. Measurements of growth were monitored by [3H]thymidine uptake and changes in cell number over a period of 3 days. Our results show that heparin—highly anticoagulant or nonanticoagulant—signifi-cantly inhibits growth of smooth muscle cells. We also show that this is a highly specific interaction with regard to molecule and cell type: i.e., other polyanions, except for a low molecular weight dextran sulfate, do not have the same effect on growth, and not all cells are inhibited by heparin; e.g., endothelial cell growth actually is enhanced. After removing antithrombin from our media, we carried out experiments which show that heparin is effective even though thrombin, a potent mitogenic agent, is still present and active. We also found that passing the platelet extract over a heparin column did not remove all of the mitogenic activity of the platelet preparation. Both experiments indicate an inhibitory role for the heparin molecule, per se. Our results support the findines of a recent paper (Guyton et al., 1980) showing that heparin can limit the size of myointimal plaques in rats after carotid injuries by inhibiting smooth muscle cell proliferation.Circ Res 47: 578-583, 1980