The dissociation constant for the binding of myosin subfragment 1 (S1) and chromatographed actin in the presence and absence of nucleotide was measured at various ionic strengths and various temperatures. The dissociation constant was of nM order in the absence of nucleotide and increased by ∼100- and ∼100,000-fold in the presence of ADP and ATP, respectively. The dissociation constant also increased with increasing ionic strength, irrespective of the presence of nucleotide, and its dependence on the ionic strength was increased by the presence of ATP but decreased by the presence of ADP. The standard enthalpy change and entropy change for the binding of S1 to actin were both positive, irrespective of the presence of nucleotide, indicating that the binding was entropy-driven. The standard entropy change was essentially unaffected by the presence of ADP but was greatly decreased by ATP, suggesting that the large increase in the dissociation constant in the presence of ATP was due to the decrease of hydrophobic Interactions. On the other hand, the increase in the dissociation constant for acto-S1 in the presence of ADP might be induced by the decrease of electrostatic interactions.