Extensive binding of 2,3 diphosphoglycerate to oxyhemoglobin is demonstrated both by diafiltration and by Sephadex gel filtration. The average concentration of both 2,3 DPGA and hemoglobin in human erythrocytes is about 5 mM; a portion of the 2,3 DPGA remains bound to oxyhemoglobin and cannot be removed by Sephadex gel filtration. © 1969.