This paper reports the survival of mouse embryos after freezing when glycol derivatives or erythritol is used as the cryoprotectant and also describes the two-step preservation of mouse embryos (rapid cooling to solid CO2 or liquid nitrogen vapor before transfer to liquid nitrogen). Ethylene glycol was the most effective cryoprotectant of glycols tested and considerable protection against freezing injury was also afforded by propylene glycol. When embryos were frozen in glycerol by a solid CO2 procedure or liquid nitrogen vapor, a relatively high survival was obtained but the survival was lower than that of embryos frozen slowly. Sucrose dilution of glycerol from the embryos frozen-thawed rapidly improved the survival rates. Survival of embryos frozen by the present two-step method was high on rapid thawing but poor on slow thawing, suggesting that the method of rapid freezing permits the formation of some intracellular ice.