We report a kinetic study of the reaction between superoxide and an important intracellular form of vitamin B-12, cob(II)alamin. Superoxide is implicated in the pathophysiology of many inflammatory diseases, whereas vitamin B,2 derivatives are often beneficial in their treatment. We found that cob(II)alamin reacts with superoxide at rates approaching those of superoxide dismutase itself, suggesting a probable mechanism by which vitamin B-12. protects against chronic inflammation and modulates redox homeostasis.