The objectives of the proposed research are as follows: 1) To correlate morphological, physiological and physical-chemical data obtained from the filament lattice of striated muscle and propose an explanation of myofilament behavior under various physiological and experimental conditions by: (a) further investigation by low-angle X-ray diffraction and, where applicable, electron microscopy of the effects of physical changes such as ionic strength, cation valency, pH, hydration and available volume upon the interaxial separation between myosin filaments in living intact single muscle fibers and skinned single muscle fibers; and, (b) extension of these studies by stroboscopic X-ray diffraction technique to include activated intact and skinned living single fibers as well as fibers in the various rigor states to determine the relationship between interaxial separation and the force vectors generated during tension generation. 2) To determine the precise liquid-crystalline nature of the myofilament lattice in striated muscle, including the origins and magnitudes of the forces which stabilize the A-band, and so define the colloidal milieu in which muscle contraction occurs. To be valid, any of the theories of muscle contraction proposed by several investigators must be compatible with the role of intermolecular forces in the structure and stability of the A-band lattice.