The actions of two natural heparins and a semi synthetic low molecular weight heparin with low anticoagulant activity have been studied on the migration and proliferation of human vascular endothelial and vascular smooth muscle cells in vitro. In a migration assay non irradiated confluent cultures of endothelial cells and smooth muscle cells were "wounded" with a sharp razor blade in such a way, that the migration of individual cells from the wound edge into a region void of cells could be measured. The maximum distance migrated (perpendicularly to the wound edge) three days after wounding, was taken as an index for migratory activity. All tested heparins reduced migration of vascular smooth muscle cells and enhanced migration of vascular endothelial cells in a concentration dependent manner. Cell proliferation was studied in clone culture. All heparins tested were found to inhibit smooth muscle cell growth. The number of clones, as well as the size of single clones, were smaller with increasing concentrations of natural and low molecular weight heparin. The endothelial cells, however, exhibited contrary responses; with increasing concentrations of heparin, the cloning efficiency and the cell number of individual endothelial cell clones increased. This opposite effects of natural heparins and low molecular weight heparin could be of importance in preventing secondary stenosing intimal proliferations after angioplasty, bypass operation and embolectomy or even atherogenesis. Since the low molecular weight heparin has only a low anticoagulatory activity, it may be more appropriate than natural heparins for long term therapy to prevent artery stenoses caused by intimal SMC proliferation.