The effects of fibroblast and epidermal growth factors on proliferation and differentiation of cultured myoblasts and chondrocytes have been compared. FGF stimulated myoblast proliferation, as determined by monitoring levels of DNA synthesis during seven days growth in vitro and by the morphology of the cultures after myotube formation. EGF has relatively little effect on myoblast proliferation. With chondrocytes, both FGF and EGF are mitogenic and FGF's, but not EGF's effect is potentiated by dexamethasone. One implication of these results is that in the course of differentiation cell types which develop from the same embryonic origin as fibroblasts are controlled by different sets of mitogenic factors. Myoblasts become primarily dependent on mitogenic agents such as FGF while chondrocytes can respond to both FGF and EGF.