Mice made tolerant to bacterial endotoxin show an increase in rate of carbon clearance from the blood and a greater stability of liver tryptophan pyrrolase following endotoxin poisoning compared with normal mice. Injections of actinomycin D, but not of ethionine nor 2-thiouracil, prevent these changes associated with tolerance. Animals exposed to 5° C cold are capable of being made tolerant despite their increased sensitivity to endotoxin. Acclimatization to cold reduces this sensitivity. Neither hemagglutinins against endotoxin coated erythrocytes nor bacterial agglutinins show any change of titer in tolerant animals. It is believed that the phenomenon of tolerance is dependent upon enzymatic stabilization.