Oxygen inhibits competitively the reduction of nitro blue tetrazolium chloride (nitro BT) by NADH and phenazine methosulfate (PMS). The oxygen-dependent inhibition is stronger in the presence of superoxide dismutase, whereas cyanide counteracts the oxygen interference. On the other hand, the oxidation of NADH mediated by PMS and dioxygen is affected only marginally by superoxide dismutase and cyanide. Therefore, it is concluded that the involvement of superoxide anions occurs at the level of nitro BT reduction via a nitro blue tetrazolinyl radical, as has been suggested by Picker and Fridovich [1984) Arch. Biochem. Biophys. 228, 155-158) and not at the level of PMS oxidation. The inhibition of the oxygen interference in the nitro BT reduction by cyanide is dependent on the cyanide concentration, whereas in nitrogen cyanide has no effect on the reduction. It is caused by competition between cyanide and oxygen to reduce or oxidize the nitro BT radical to either formazan with concomitant cyanogen production or nitro BT, respectively. For the histochemical localization and analysis of electropherograms of NAD(P)+-dependent dehydrogenase activities, the interference of oxygen can be avoided by anaerobic incubations or by the use of 5 mM nitro BT when incubating aerobically.