AbstractMagnitude of cooperativity of a hypotheticalm‐subunit allosteric system of hemoglobin type is discussed, based on computation of the amount of O2to be transferred to the corresponding monomeric nonallosteric system of myoglobin type. For quantitative discussion, “allosteric gain” is defined as the difference between the amount of O2to be transferred from the allosteric to the nonallosteric system and that from the hypothetical system of average O2affinity to the nonallosteric system. These two quantities, allosteric gain and amount of O2transfer, are quite sensitive to equilibrium constant ratio, especiallyKm/K1, (Ki+1/Ki)max, andi.Hill's coefficient appropriately describes neither the amount of O2transferred nor allosteric gain. Allosteric gain or the amount of O2transfer which is best characterized by the total cooperativity index (Km/K1) and the local cooperativity indices (Ki+1/Ki) seem to be appropriate measures of the magnitude of cooperat