When Wyatt and Kalf (in press) reported the presence of glycerol in the haemolymph of Hyalophora cecropia (L.) pupae, the possibility that it was responsible for the ability of this stage to survive freezing was naturally raised. It is well known that glycerol may protect animal tissues from injuries otherwise caused by ice formation. Many other substances have a degree of similar protective ability but are less effective for various reasons. Lovelock (1954) tested the protective action of 15 neutral solutes on human red blood cells and found glycerol to be almost ideal because of its high solubility in aqueous salt solutions, its ability to permeate the cell rapidly and completely, its low molecular weight, and its absence of toxicity even when in great excess.