Vitamin E, supplemented to semisynthetic or natural commercial diets in amounts of 60–180 mg/kg increased the humoral immune response of mice against sheep red blood cell or tetanus toxoid antigens by 30–40%, measured by the antibody plaque forming cell test, or by hemagglutination. Vitamin E affected particularly the IgG antibody production. The antioxidant NN-diphenyl-p-phenylene diamine was ineffective in restoring immune response in a vitamin E-deficient diet, and was only partially effective in replacing vitamin E in a normal diet.