The two subunits of tropomyosin in cat fast and slow and human vastus lateralis muscles were electrophoretically identical, while the slower migrating cardiac tropomyosin unit was different. One troponin protein, band 5a, was electrophoretically different when cat fast and slow and cat heart and slow muscles were compared. There were no qualitative differences in the electrophoretic patterns of tropomyosin and troponin from normal and Duchenne dystrophic human skeletal muscle.