At a concentration of 24 μM, azide did not affect the endogenous, ammonia-dependent, and acetate-dependent respiration rates but did instantaneously and completely inhibit nitrite oxidation (Table 1 and Fig. 2). In addition, the inhibition was independent of the nitrite concentration and was reversible after azide was removed by biomass washing (data not shown). This is the first report in which azide is described as an inhibitor of nitrite oxidation in vivo (50% inhibition at a concentration of 0.3 μM) (Fig. 2). In vitro, azide completely inhibited nitrite oxidation in cell extracts ofNitrobacter agilis at a concentration of 100 μM, although lower concentrations were not studied by the authors who performed this study (1). Previous results for the purified nitrate reductase of a denitrifying bacterium showed similar inhibition (13). As the nitrite oxidoreductase system is able to act as a nitrate reductase in the absence of oxygen (i.e., nitrate is transformed to nitrite) and both enzymatic systems contain a molybdenum cofactor (6, 8, 13), we assume that azide could act by complexation with the molybdenum atoms of the nitrite oxidoreductase.