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 μU./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 < 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 < 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 < 0.005 and 2 p < 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.

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