O-.2 production is the first step in the generation of a group of powerful microbicidal oxidants by neutrophils. The production of O-.2 is catalyzed by a membrane-bound, NADPH-preferring flavoprotein oxidase, a conclusion supported by much evidence including the discovery of a new form of chronic granulomatous disease caused by a mutation affecting that oxidase directly. Also involved in the O-.2-forming reaction is a b-type cytochrome; the role of this cytochrome is as yet undefined, though it does not appear to be on the direct route of electron transfer between NADPH and oxygen. It has been postulated that quinones too participate in the O-.2-forming reaction, but further work is necessary to define their role more fully.