Troponin activity of natural actomyosin in various muscle diseases was determined. The activities were expressed as the trypsin-sensitive calciumbinding of natural actomyosin. Among various muscle diseases, muscular dystrophy, especially the Duchenne type, showed greatly decreased values. These decreases may be directly relevant to the dysfunction of the affected muscle. In female carriers of Duchenne dystrophy, one showed normal and the other showed slightly decreased values. Polymyositis, thyrotoxic myopathy, myotonic dystrophy, and paramyotonia congenita revealed normal troponin activities. Nemaline myopathy showed slightly decreased values. Troponin activity was fairly well preserved in advanced neurogenic atrophy.