Skeletal muscle is a tissue that adapts to increased use by increasing contractile protein gene expression and ultimately skeletal muscle mass (hypertrophy). To identify hypertrophy-inducing agents that may be potentially useful in the treatment of age-related muscle loss (sarcopenia) and to better understand hypertrophy signal transduction pathways, we have created a skeletal muscle cell–based hypertrophy–responsive system. This system was created by permanently modifying the relatively undifferentiated C2C12 cell line so that it contains the β-myosin heavy chain (β-MHC) gene promoter and enhancer regions fused to a luciferase reporter gene. This cell line responds, by increasing luciferase expression, to a variety of skeletal muscle hypertrophy–inducing agents, including insulin, insulin-like growth factor I, testosterone, and the β-adrenergic receptor agonist isoproterenol, in both the undifferentiated and differentiated states. This cell-based system should be useful for identifying novel hypertrophy-inducing agents as well as understanding hypertrophy signal transduction.