This study concerns a mouse strain which is mutant with respect to blood and tissue catalase. The mutant catalase is abnormally sensitive to heat and extraordinarily sensitive to mild alkalinity. Antisera prepared by immunizing rabbits to the blood of normal mice stabilize the mutant catalases against the effect of heat and alkaline pH. As a result of this stabilization, the antigen-antibody complex exhibits a greater catalatic activity than was originally added, i.e., an apparent activation.