Abstract: Compositions and methods for stimulating muscle growth or differentiation in a vertebrate are disclosed. Such compositions include a muscle-trophic amount of interleukin-15 and can be used to treat a variety of conditions including disuse atrophy, wasting, various age-related disorders, secondary effects of diabetes, including glucose-intolerance, as well as muscular dystrophy, rhabdomyosarcoma and congestive heart failure. The compositions and methods of the invention find agricultural use in increasing the efficiency of meat and milk production of farm animals.