Although it is recognized that red cells lose membrane during storage, estimation of the osmotic fragility of erythrocytes has not previously proven to be a useful measurement of the storage lesion. Erythrocytes from blood stored in CPD-A2 were found to have a markedly increased osmotic fragility. A major portion of this increase was found to be due to accumulation of lactate, which is only slowly transported from within erythrocytes and which therefore exerts a strong osmotic effect in the usual osmotic fragility test. After an hour's incubation in a large volume of iso-osmotic buffer, the osmotic fragility curve of stored erythrocytes was much more nearly normal. Such cells were found to have a volume 5%--8% greater than that of normal cells, indicating that even after removal of lactate more osmotically active material was present in the stored erythrocytes than in fresh cells. Most of this differences can be accounted for by substitution of chloride ion for 2,3-DPG, since chloride exerts approximately 3.7 times the osmotic effect of 2,3-DPG per unit charge. In addition to the shift in osmotic fragility produced by the increased intracellular osmotically active material, a “fragile tail” of red cells was also present. Stored erythrocytes were labeled with 51Cr and reinfused into the volunteer donors. The osmotic fragility of the reinfused cells was estimated using a technique of sequential osmotic hemolysis that permitted accurate estimation of osmotic fragility of transfused cells using very small amounts of 51Cr. The osmotic fragility of the reinfused cells became less than those of fresh cells after 24 hr and was exactly the same as those of fresh cells after 4 days. The fragile tail disappeared at a rate that approximated the rate of loss of nonviable erythrocytes from the circulation as measured by 51Cr. These findings are consistent with the preferential destruction of a subpopulation of red cells with a diminished surface area.