Erythrocytes stored in an acid-citrate-dextrose preservative (ACD) at 4 to 7°C. undergo morphological and biochemical alterations. The cell becomes more sphere-shaped with an associated increase in osmotic and mechanical fragilities, the glycolytic rate and organic phosphate compounds are decreased, inorganic phosphate is increased, and the differential between cell potassium and sodium is decreased (1). These altered cells, when transfused to a recipient, are destroyed in an amount dependent upon the duration of storage of the blood. Thus, storage for three weeks in ACD results in a viability 2 of about 85 per cent and for four weeks in a viability of about 60 per cent (2, 3).Since the normal human erythrocyte population is destroyed in vivo at a rate of 0.8 to 1.0 per cent per day, the survival of erythrocytes after three to four weeks of storage is consistent with the expected in vivo loss from aging. The present study was undertaken to determine whether the changes of storage were related to a normal or accelerated aging process, or whether they represented cell damage due to in vitro storage conditions.