Equilibrium titrations and kinetic experiments were used to define the cooperative binding of myosin subfragment 1 (S1) to actin-troponin-tropomyosin. Both types of experiment require an equilibrium between two states of the thin filament in which one state (the off state) binds S1 less readily than the other. Equilibrium titrations are compatible with > 95% of the actin7.Tn.Tm units being in the off state in the absence of calcium and 80% in the off state in the presence of calcium. Kinetic binding data suggest that the presence of calcium switches the thin filament from 70% in the off state to < 5%. The two experiments, therefore, define quite different populations of the off states. We propose a three-state model of the thin filament. A "blocked state" which is unable to bind S1, a "closed state" which can only bind S1 relatively weakly and an "open state" in which the S1 can both bind and undergo an isomerization to a more strongly bound rigor-like conformation. The equilibrium between the three states is calcium-dependent; KB = [closed]/[blocked] = 0.3 and > or = 16 and KT = [open]/[closed] = 0.09 and 0.25 in the absence and presence of calcium, respectively. This model can account for both types of experimental data.