Smooth muscle cells were grown from thoracic aortas of 1-year-old monkeys (Macaca nemistrina). The effect of insulin on the proliferation of these cells was studied by comparing the growth of cells in culture medium to which insulin had been added with that of cells in basal (1% monkey serum) medium and in growth-promoting 5% monkey serum. Insulin in concentrations of 10, 100, 1,000, and 10,000 muunits/ml resulted in successively greater stimulation of growth which was highly significant (P smaller than 0.001) by analysis of variance. There was a significant linear relationship between the logarithm of the insulin dose and cell growth. However, the highest concentration of insulin produced only 50% of the effect of 5% monkey serum. Serum from which insulin had been removed stimulated growth less well (P smaller than 0.05) than did untreated serum at the same concentration (5%) but had significant (P smaller than 0.05) stimulating properties compared with whole serum at a lower concentration. Cells that were older in culture life (eight or nine passages) did not show a growth response to insulin and had an attenuated response to 5% serum. The effect of insulin (100 muunits/ml) was inhibited by dibutyryl cyclic adenosine monophosphate (db-cAMP) (5 times 10-5 M), although there was a latent period of 3 days before inhibition occurred; db-cAMP had no effect on cell counts in the absence of insulin. The electron microscopic appearance of the cells was unaltered by insulin.