Abstract 1. 1. Using the immunoperoxidase staining technique with monospecific antibodies to the polymorphic forms of the components of the troponin complex, two types of cells could be distinguished in rat leg muscles at 18 days' gestation. 2. 2. One kind of cell, designated presumptive type II, stained only with the fast skeletal muscle forms of troponin I, troponin T and troponin C. 3. 3. The remaining cells designated presumptive type I, stained with both the fast and slow skeletal muscle forms of the three troponin components. 4. 4. The amounts of the fast skeletal muscle forms of the troponin components progressively decreased in the presumptive type I cells until 10–11 days after birth these proteins could no longer be detected by the immunoperoxidase staining. At the same time the slow skeletal muscle forms of the troponin components steadily increased. These changes were complete about a day earlier in muscles containing a majority of type I cells. 5. 5. Differentiation into different muscle cell types during development could be distinguished earlier by staining with the troponin antibodies than by the classical histochemical methods. 6. 6. The results in general support the hypothesis that the genes responsible for synthesis of the components of the troponin complex are under some type of coordinated control.