Earlier studies have shown a stimulatory effect of diabetic serum on the growth of rabbit aortic medial cell cultures. Growth media supplemented with normal serum with added insulin (50-2,000 muU./ml. serum) did not enhance the growth of the medial cell cultures. Control media containing serum from recent diabetics with low insulin concentration stimulated the growth (2p less than 0.01). Supplementation of normal serum with human growth hormone (final concentration 1-5 ng./ml. medium) resulted in a significant enhancement of growth (2p less than 0.005). The growth-promoting effect of growth hormone was not detectable with lower concentrations (0.5 ng. and 0.1 ng./ml. medium). The growth effect of the low concentration of growth hormone could not be augmented by increasing the concentration of glucose in the incubation medium. Growth hormone in an amount of 1 ng./ml. medium increased both the number of 3H-thymidine-labeled cells as identified by autoradiography and the number of mitotic bodies (2p less than 0.005 and 2 p less than 0.025). The present results demonstrate that the growth-stimulating factor(s) in diabetic human serum described earlier is not insulin but may well be growth hormone.