Homogenates of normal human granulocytes were found to catalyze the oxidation of epinephrine to adrenochrome. This reaction was abolished by superoxide dismutase and catalase, but not by albumin or boiled dismutase, indicating that epinephrine oxidation was dependent on O2- AND H2O2. Elimination experiments to identify the electron donor for O2- production showed that the reaction was not inhibited by gel filtration of the homogenate or by removal of glucose, sucrose, or phosphate from the reaction mixture, raising the possibility that epinephrine itself was the reducing agent for the production of O2-. However, we could obtain no evidence for the direct involvement of epinephrine in this step. To explain our observations, we have proposed a mechanism of adrenochrome production involving a radical chain process with one or more enzyme-catalyzed steps. Reactants participating in this chain were postulated to be O2-,-OH (produced by the reaction of O2- with H2O2), and various intermediate products of oxidation of epinephrine. An unidentified endogenous constituent was postulated as the agent responsible for the initial conversion of oxygen to O2-.