Injection of mouse marrow preserved in glycerol at -80 C. for six weeks has been shown to be quantitatively similar to injections of fresh marrow in inducing recovery in mice after lethal radiation. The effect of the preservation procedure on subsequent rates of DNA synthesis has been determined in vitro for both preserved human marrow and preserved mouse marrow. Since DNA synthesis is a satisfactory measure of rates of cell multiplication and since preservation is shown to affect DNA synthesis in human marrow to a lesser degree than in mouse marrow, it may be concluded that the procedure used (-80 C. plus 15% glycerol) is satisfactory for the maintenance of viability in human marrow.