Ninety-four units of blood have been collected, glycerolized, stored in the frozen state at either —80 C or —120 C, deglycerolized, stored at 4 C, and then used clinically. A technique has been developed for maintaining sterility at every step. Even after storage at very low temperatures for periods up to 19 months, the red blood cells appeared therapeutically comparable to cells stored at 4 C up to 21 days in a standard A.C.D. (anticoagulant acid citrate dextrose) solution. The longevity of red blood cells in the circulatory system of the recipient was studied by using chromium 51 for labeling and was found to be independent of the duration of storage. The loss of red blood cells by lysis during the process as a whole seemed to be due entirely to irreversible random damage during collection and handling rather than to storage at low temperature. The sterility of the system made possible an additional postthawing storage of the red blood cells for up to 11 days at 4 C before use in transfusions. No transfusion reactions of any kind were noted.