The synthesis of contractile proteins has been studied during the differentiation of quail skeletal muscle myoblasts in culture. Myoblast differentiation was synchronized by transferring secondary cultures of rapidly dividing myoblasts into medium lacking cell division-promoting factors. Cultures at various stages of differentiation were then pulse-labeled with 35S-methionine, and cell extracts were resolved by electrophoresis on two-dimensional gels. Incorporation into specific proteins was quantitated by autoradiography and fluorography using a scanning densitometer. Contractile proteins synthesized by muscle cultures were identified by their co-electrophoresis on two-dimensional gels with contracile proteins purified from quail breast muscle. Our results show that the synthesis of myosin heavy chain, two myosin light chains, two subunits of troponin and two subunits of tropomyosin is first detected at the time of myoblast fusion and then rapidly increase at least 500 fold to maximum rates which remain constant in muscle fibers. Both the kinetics of activation and the molar rates of synthesis of these contractile proteins are virtually identical. Muscle-specific actin (alpha) synthesis also increases at the time of myoblast fusion, but this actin (alpha) is synthesized at 3 times the rate of other contractile proteins. The synthesis of 30 other muscle cell proteins was quantitated, and most of these are shown to follow different patterns of regulation. From these results, we conclude that the contractile proteins are regulated coordinately during myoblast differentiation.