The two-state model of Monod, Wyman and Changeux has been used extensively in analysis of the functional behavior of human hemoglobin. Within the context of this model, the cooperativity, pH dependence of oxygen binding, and anionic regulation of oxygen affinity are all considered to be due to shifts in the equilibrium between low affinity (T) conformational states and high affinity (R) confor-mational states. The heterotropic effectors, such as protons, carbon dioxide, inorganic anions, and organic polyphosphates, are considered to be principally active in causing shifts in the alloste-ric equilibrium constant L. For both simple anions, such as chloride, and strong effectors, such as inositol hexaphosphate, the allosteric equilibrium constant L increases with increasing concentrations of the effector.