Normal aortic endothelial cells cultured in serum-free medium elaborate a factor(s) which cause the proliferation of smooth muscle cells or 3T3 cells grown in medium containing plasma-derived serum. For smooth muscle cell growth plasma factors are required in addition to the endothelial cell-conditioned medium whereas 3T3 cells will grow in the presence of endothelial cell-conditioned medium alone. Injured, growing cultures of endothelial cells do not appear to elaborate any more of the mitogen than normal, quiescent endothelial cells. The mitogenic factor is as yet poorly characterized but is heat stable at 56 degree C for 30 minutes. These results suggest that endothelial cell-smooth muscle cell interactions may contribute to the intimal smooth muscle cell proliferation which is a characteristic of the atherosclerotic lesion.