The oxygen consumption, superoxide production and hydrogen peroxide generation was studied in human neutrophils phagocytosing zymosan particles. Application of sodium azide, as an inhibitor of catalase, and/or 1,3-bis(chloroethyl)-1-nitrosourea (BCNU), as an inhibitor of glutathione reductase, led to the conclusion that neutrophils convert about half of the oxygen consumed in the respiratory burst to hydrogen peroxide; the other half is used for formation of organic peroxides, disulfide bridges, etc. These products are rapidly degraded to water by catalase and/or the glutathione redox cycle. Reduction of exogenous cytochrome C accounted for only about 15% of the consumed oxygen. Neutrophil homogenates contain a badly damaged oxidase system, because oxygen consumption and hydrogen peroxide formation were only about one-tenth of that observed with whole cells. In contrast, cytochrome-C reduction was about three times as high as that found with intact cells. Probably, cytochrome C partly reconstitutes damaged oxidase systems, thus artificially increasing the oxidase activity. We conclude that cytochrome-C reduction is not a good parameter to characterize cell-free oxidase preparations.