Stocks of mutant mice have been reestablished from eight-cell embryos stored in liquid nitrogen for varying periods up to 11 years, and no evidence has been found of deterioration of survival with time of storage. Also, studies on the simulated cumulative effect of background radiation during storage failed to find any detrimental effect when embryos were exposed to the equivalent of about 2000 years of background radiation. However, in some cases embryos that carry mutant genes or chromosome anomalies tend to survive the freezing and thawing procedure less well than F1 hybrid embryos. Although this effect is probably independent of storage time, recent improvements in technique upon embryonic survival are to be welcomed.