Abstract: The functional properties of FGF10 during avian skeletal muscle development in vitro and in vivo are currently unknown. To elucidate such a function, the hindlimb skeletal muscle and primary muscle cultures of chick embryos were inoculated with high-titer RCAS-FGF10 retrovirus. Ectopic FGF10 stimulated a 4-fold increase in primary muscle cell proliferation and differentiation (p<.05), thereby significantly enhancing primary skeletal cell growth and development in vitro. In contrast, ectopic FGF10 expression had no significant effect on skeletal muscle development in vivo. These results suggest that FGF10 is not a primary growth factor involved in stimulation of the initial events of avian skeletal muscle development in vivo, and demonstrate the importance of confirming growth factor activity in vitro with its properties in vivo.