1. Difference spectra, at room and liquid N 2 temperatures, of S 2O 4 2−-, and NO 2 −-reduced intact cells and cell-free preparations of Nitrobacter agilis demonstrated the presence of cytochromes of the c- and a-types. Reduction of cytochromes by succinate, and to a limited extent, by NADPH also occurred, provided KCN (0.1 mM) was also present. 2. A particulate, heat-labile nitrite oxidase having an absolute requirement for O 2 was prepared from N. agilis cells using sonic oscillation and differential centrifugation. The particles also possessed NADH oxidase, succinoxidase, formate oxidase and traces of NADPH oxidase activity. The stoichiometry of the nitrite oxidase reaction approached the theoretical value of 2 moles of NO 2 − consumed per mole of O 2 consumed. The pH optimum of the nitrite oxidase system shifted to progressively more alkaline values as the NO 2 − concentration was increased, changing from a pH value of 6.8 at 0.6 mM KNO 2 to pH 8.0 at 0.01 M KNO 2 with apparent K m 's of 0.2 and 1.2 mM NO 2 −, respectively. Computations of the HNO 2 concentrations present under the above conditions showed an approx. 500-fold greater affinity for HNO 2 which was independent of pH, suggesting the involvement of HNO 2 as both a substrate and an inhibitor (at higher concentrations) of the nitrite oxidase system. The marked inhibition by NaN 3, NaCN and Na 2S, as well the light-reversible inhibition by CO, indicated the presence of cytochrome oxidase which was subsequently characterized. NO 2 − proved to be a competitive inhibitor of the nitrite oxidase system. 3. The particulate preparation also possessed a heat-labile nitrite-cytochrome c reductase activity which was energy independent and routinely measured under anaerobic conditions. As in the case of nitrite oxidase, the affinity of the enzyme for NO 3 − increased as the pH was lowered, but the pH optimum remained unaffected. In terms of calculated HNO 2 concentration an approximately constant K m of about 0.2 μM was estimated at the several pH's examined. The inhibition by NO 3 − was shown to be competitive. The marked sensitivity of the reductase to several metal-binding agents implicated a metal component in the electron transport chain at the site prior to cytochrome c. 4. The membrane-like composition of the nitrite oxidase system is indicated.