Storage of human erythrocytes in SAG media results in the release of membrane microvesicles with a diameter of about 190 nm. They can be separated from intact erythrocytes by centrifugation on a dextran barrier solution (density 1.078 g/l). Vesicles prepared from cold stored erythrocyte concentrates are like those released upon ATP depletion by erythrocytes incubated without glucose at 37 degrees C [15]. The course of vesiculation was followed by measuring acetylcholinesterase during storage of the erythrocyte concentrate for 35 days. Its activity remained constant within the storage units during the preservation period. This enzyme and phospholipids were released continuously in a proportional manner. The release of sialic acid amounted to about half of that of phospholipids. Owing to depletion of 2,3-bisphosphoglycerate the binding of ATP to haemoglobin increased and the concentration of free ATP declined. Addition of an ion-exchange resin to stored erythrocytes kept the pH constant, retarded the breakdown of 2,3-P2G and stabilized the concentration of free ATP. That inhibited the rate of irreversible vesiculation. Therefore, maintenance of 2,3-bisphosphoglycerate plus ATP during long-term storage of erythrocytes is a condition of keeping intact their membrane, metabolism and oxygen transport function.