Cytochrome cd of Alcaligenes faecalis, which has been found to be a nitrite reduc-tase, was found to have a NO-reducing activity which converts NO to N2O, when ascorbate-phenazine methosulfate was used as an electron donor system. Some characteristics of the reduction of NO by cytochrome cd were examined, comparing with those by the particulate-bound NO-reductase. (1) The specific activity was about 500 μ1 NO-uptake (or 250 μ1 N2O-output)/hr/mg of cytochrome cd at pH 7 and at 10% NO in the gas phase of the reaction vessel. (2) The optimum pH for the reduction of NO by cytochrome cd was 5.5, which is similar to that by the particulate-bound NO-reductase. (3) The NO-reducing activity of the particulate-bound enzyme was inhibited by more than 20% of NO in the gas phase, while that of cytochrome cd was not saturated even at 60% of NO in the gas phase. (4) Effects of some inhibitors were examined upon the reduction of NO. Cyanide, azide or diethyldithiocarbamate inhibited the reduction of NO by particulate-bound NO-reductase stronger than that by cytochrome cd.