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We used light scattering and ultracentrifugation to study the binding ratio of myosin and actin from normal and failing dog hearts and to determine the effect of temperature and adenosine triphosphate (ATP) on the binding. For comparison, similar studies were done on myosin and actin from rabbit skeletal muscle. We found a higher combining ratio, by weight, for cardiac myosin and actin (4.8 : 1) than for skeletal myosin and actin (4.4 : 1). The dissociation constants were similar at 24°C for cardiac and skeletal proteins. But at 10°C, the dissociation constant was greater for cardiac proteins, and the difference appeared more pronounced when phosphate was used to buffer the solutions. The dissociation constant at 10°C was greater for myosin and actin from failing dog hearts. ATP caused dissociation of cardiac and skeletal F-acto-heavy meromyosin and F-actomyosin. There was evidence that the binding of cardiac proteins may not be as tight as that of skeletal muscle proteins.