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The binding of actin to heavy meromyosin (HMM) The sliding filament theory of muscle contraction proposes that contraction is caused by the cyclic formation and breakage of links between actin and myosin filaments, accompanied by the hydrolysis of ATP (1). One of the key questions in the biochemistry of muscle is, therefore, the nature of the interaction of actin, myosin, and ATP in vivo. Under in vitro coinditions, at low ionic strength, both F-actin and myosin are filaments and, therefore, the interaction between the two proteins is very difficult to study quantitatively. (9-11). Third, in a kinetic study at 6VC, the data suggested that one H.II\ molecule binds to one actin monomer in contrast to the one-to-two ratio observed in the absence of ATP (12).In the light of these differences in the interaction of actin with HMTM in the presence and absence of ATP, we have investigated this interaction in the presence of ATP using both ATPase (ATP phosphohydrolase EC 3.6.1.3) studies and analytical ultracentrifugation.