In order to determine the feasibility of 4 C storage of frozen‐thawed red blood cells beyond the 24 hours now allowed by the Food and Drug Administration (BoB), chemical and cytological measurements were made of blood units processed by different methods. The only significant findings were a progressive increase in the plasma hemoglobin and plasma potassium. These were most pronounced when the storage medium was the 0.8% NaCl‐0.2% dextrose and storage was continued past five days. Storage in autologous plasma produced less pronounced changes, but the changes were still about twice that found in a unit of whole blood stored for the same amount of time. Storage in 5% albumin or Plasma Protein Fraction did not seem to provide conditions much superior to the dextrose‐saline solution. Contamination of the unit produced somewhat more hemolysis than in its sterile control but not enough to be readily detectable. From these studies it seems logical that the 4 C storage time could be extended to at least three, and in all probability five days, with no significant harm to the recipient, and with a great increase in the useable life of the blood unit, thus increasing the availability of a very precious comm