A nitrite reductase [EC 1. 7. 99. 3] was isolated from Achromobacter cycloclastes and characterized. The oxidized form of the enzyme had absorption maxima at 283, ∼400 (a shoulder), 464, 590 and 700 mμ; the reduced form had no peak in the 400–800 mμ region. The molecular weight was 69,000. It had two copper atoms per molecule. The enzyme catalyzed nitric oxide production from nitrite, whereby most part of the nitrite-nitrogen was converted to nitric oxide gas. Optimum pH of this reaction was 6.2. Km for nitrite was estimated to be ∼5×10−4M. This reaction was inhibited by KCN, diethyldithiocarbamate, p-chloromercuribenzoate and CO. Nitrous oxide production from nitrite and hydroxylamine was also catalyzed by the enzyme.