Abstract
1. Myofibrils from human skeletal muscle contained regulatory proteins exhibiting similar electrophoretic behaviour to those present in rabbit skeletal muscle. 2. All human skeletal muscles examined contained two forms of troponin I corresponding to the forms already characterized in fast and slow rabbit muscle. 3. The ratios of the amounts of the two forms of troponin I in different human skeletal muscles were not identical with the ratios for the type 1 to type 2 fibres published in the literature. The ratios could, however, be arranged in the same rank order. 4. Primate heart contained a single form of troponin I different in molecular weight and amino acid composition from the skeletal forms. 5. A monospecific antiserum to human cardiac troponin I was prepared in the sheep and shown not to react with the fast or slow skeletal-muscle forms of troponin I from human and other species. 6. The anti-(human cardiac-muscle troponin I) reacted with the cardiac troponin I from the human, baboon, rabbit and rhesus monkey. Positive reactions were also obtained with urea extracts of whole cardiac tissue.