Studies are in progress in the following areas, in elaboration of previous work from this laboratory: 1. Subunit structure of myosin: Characterization of subunit composition and interaction of myosin by ultracentrifugal, electrophoretic, and other physical chemical methods. Comparative studies on skeletal and cardiac myosin from rabbit, human, and other vertebrate muscle. Studies on possible changes in myosin related to cardiac hypertrophy and aging. 2. Myosin-ATP interaction: Hybridization experiments on subunit specificity of myosin ATPase, binding of nucleotide to myosin subunits, kinetic analysis of pressure and temperature effects on myosin ATPase. 3. Ultrastructural studies of relaxed and contracted muscle.