It is now well established that myosin, of molecular weight 470,000(1) is comprised of an elongated helical core of two heavy chains, each of molecular weight 210,000, containing two to three light chains (2) of molecular weight 20,000 (1). By a variety of methods (3) the light chains may be dissociated from the heavy chains witharesultant loss in the ATPase and actin combining properties of the molecule. The light chains have also been shown to be present in subfragment-1, the biologically active fragment of myosin produced by papain digestion(4). Each mole of subfragment-l(mol. wt. 105,000) has been shown to contain one light chain (5).

These earlier studies suggested that some functional role might exist for the light chains at or near the active site of the molecule and the evidence to be presented here shows that the presence of both light and heavy chains and their interaction are a necessary prerequisite for the hydrolysis of ATP by myosin and for its interaction with F-actin. For these studies the dis-* Career Scientist of the Health Research Council of the City of New York.