The composition of NADPH oxidase purified by Red Sepharose chromatography of extracts from human neutrophil membranes was investigated. In contrast to that was recently reported by others, the enzyme isolated according to this procedure contained a high concentration of cytochrome b - 2 4 5 and little FAD. The results reinforce the belief that cytochrome b - 2 4 5 is a major component of the NADPH oxidase and plays a fundamental role in the formation of O 2 - by neutrophils.