SummaryBovine erythrocytes can be preserved for long periods of time by freezing and storing in liquid nitrogen. Blood group determinations, titrations, and isoimmunizations indicated that there were no detectable alterations in antigenic reactivities of preserved erythrocytes when compared with fresh samples of blood from the same animal. In addition, consistent percentages of recovery within a frozen mixture could be obtained, indicating that the deterioration of erythrocytes with time was not significant. Variations in the percentage of recovery of erythrocytes with different concentrations of sucrose (the cryoprotective agent) were significant at the 0.01 level of probability. The highest average percentages of recovery of erythrocytes from whole blood and the cellular fraction of blood (blood from which plasma was removed before freezing) were obtained with 45 and 40% sucrose, respectively. Results also indicated that whole blood gave a slightly higher percentage of recovery, than the cellular fraction. The individual donor effect, storage time in liquid nitrogen, and various saline concentrations of the thawing and washing solutions had no significant effects upon percentage of recovery.