Previous studies have shown that growth hormone promotes the differentiation of preadipose 3T3 cells into adipose cells. This action of the hormone is not mediated by insulin-like growth factor 1 (IGF-1), but is exerted directly on the preadipocytes. In order to determine whether growth hormone promotes the differentiation of other mesenchymal cell types, we have examined its effect on the formation of multinucleated muscle cells from myoblasts. Clone mu cells, generated from line 10T1/2 by treatment with azacytidine, were known to be capable of undergoing myogenesis. When these cells were cultivated in a medium not able alone to support their differentiation, the addition of physiological concentrations of growth hormone strongly promoted myogenesis. When fetal calf serum was used instead of growth hormone to promote myogenesis, the effectiveness of the serum was reduced by the addition of a specific antiserum to growth hormone. IGF-1 could not substitute for growth hormone in promoting myogenesis. When another 10T1/2 line not ordinarily able to form myotubes was subjected to treatment with 5-azacytidine, it was subsequently able to give rise to muscle and adipose cell colonies; the number of such colonies was much increased by the presence of growth hormone. We have not been able to demonstrate a similar dependence on growth hormone for myogenesis by the established lines L6E9 and C2.