The expiration of thawed red cells (RBC) currently is 24 hours. This leads to problems in current transfusion practice, and often wastes rare and costly products. To remedy this situation, we developed a synthetic medium containing 0.085 g per I adenine, 5 g per l glucose, 48.0 g per l sucrose, 0.75 g per l NaH2PO4 .cntdot. H2O, and 2.5 g per l NaCl, with a pH of 7.40 and an osmolarity of 320 mOsm. After 15 days post-thawing storage at 4.degree. C in our medium, the viability and function of RBC were maintained. We propose the storage of thawed RBC at 4.degree. C in our medium for up to 9 days: sterility was maintained, pH was 6.80, and 2,3-diphosphate glycerate was 50 percent, and adenosine triphosphate 100 percent of the original values. Free hemoglobin was 122 mg per unit, adenylate energy charge was 0.90, and RBC labeled 7 days after thawing showed 89 percent survival 24 hours after transfusion and a one-half disappearance of 22 days.