In culture fibroblasts, endothelial cells and smooth muscle cells from arterial wall are stimulated by thrombin and factor XIII, whereas fibronectin inhibits the proliferation of these cells. Experiments with tumor cells show growth stimulation of sarcoma cells and HeLa cells by thrombin and factor XIII. While growth of sarcoma cells is inhibited by fibronectin, HeLa cells are stimulated by fibronectin in physiological concentrations. Possible mechanisms of these effects and relations to in vivo-phenomena are discussed.