Stimulation of the growth of muscle cells in tissue culture by hormones with established anabolic actions in whole animals will be investigated. The hormones to be studied are testosterone, growth hormone, insulin, and somatomedin. In addition to direct effects of these hormones on muscle cells, interactions among the hormones will also be investigated - i.e., is insulin required for the growth- promoting action of growth hormone in muscle? In those cases in which direct effects of the anabolic hormones are observed, they will be compared with presence, absence, or change in concentration of hormone receptors on the surfaces of the muscle cells. Emphasis will be placed upon correlation of the effects of the hormones in tissue culture and established levels and rates of hormone effects in animals.