Studies on the thermal fragility of red cells of different spp. of animals at various temperatures reveal the rat erythrocyte to be the most heat-sensitive, followed in order of decreasing sensitivity by those of the rabbit, guinea pig and man. A correlation exists in these spp. between the red cell count and degree of termal sensitivity. Some effects of mechanical trauma and ether in increasing termal fragility and decreasing osmotic fragility are presented. Studies on red cells tagged with radioactive Fe in-dicate that young cells are more resistant to thermal hemolysis than are older cells. Sickle cells were resistant to termal fragility.