A single injection of alloxan¹, or alloxan-like derivatives of uric acid^2,3, kills the insulin-producing islet β-cells and causes diabetes mellitus in animals. This effect is probably mediated by a sequence of redox reactions involving the production of superoxide anion radicals in or near the β-cells^4–8. Superoxide anions may also arise in inflammation^9–15, and islet inflammation often accompanies the outbreak of human diabetes¹⁶, perhaps following the combined onslaught of viruses and chemical agents on the β-cells¹⁷. We now report that injections of superoxide dismutase, an enzyme removing superoxide anion radicals, act prophylactically against alloxan-induced diabetes.