Glycolysis in human red blood cells is enhanced by deoxygenation of the suspension. Accumulation of 2, 3-diphosphoglycerate is observed under this condition. Cross-over analysis of glycolytic intermediates during the transition of red blood cells from de-oxygenated to oxygenated state reveals phosphofruktokinase (EC 2. 7.1.11) and pyruvate kinase (EC 2. 7.1. 40) as cross-over points. The following two mechanisms are discussed to explain the changes in the metabolic pattern: i) Release of proton by oxygenation of hemoglobin, and ii) release of 2, 3-diphosphoglycerate from hemoglobin by oxygenation. It is concluded that, under physiological conditions, the oxygen affinity for hemoglobin may be controlled by the change in 2, 3-diphosphoglycerate concentration.