In mice with alloxan-induced diabetes, humoral and cellular immunological reactivity were weak. The number of leucocytes, and especially lymphocytes, was reduced, and the weight and cellularity of lymphatic organs were lower than in normal mice. Treatment of diabetic mice with insulin reversed morphological and functional deficiency of the immunological system. Observed depression of immunological functions was attributed to impaired transport of glucose into immunocompetent cells, rather than to toxic effects of alloxan.