Dissimilatory nitrite reductase was isolated from extracts of Alcaligenes xylosoxidans subsp. xylosoxidans (N.C.I.M.B. 11015), after activation of crude extracts by the addition of copper(II) sulphate. The enzyme was purified by a combination of (NH4)2SO4 fractionation and cationic-exchange chromatography to 93% homogeneity as judged by SDS/PAGE. SDS/PAGE and spray m.s. showed that the enzyme had a subunit M(r) of 36.5 kDa. The copper content was 3.5 +/- 0.8 Cu atoms/trimer of M(r) 109,500. E.p.r. spectroscopy of nitrite reductase as isolated showed that both type 1 (g parallel = 2.208, A parallel = 6.3 mT) and type 2 (g parallel = 2.298, A parallel = 14.2 mT) Cu centres were present, in contrast with published data [Masuko, Iwasaki, Sakurai, Suzuki and Nakahara (1984) J. Biochem. (Tokyo) 96, 447-454], where only type 1 copper centres were reported. Our preparations had a specific activity of 150-300 mumol of NO2- reduced/min per mg of protein, 6-12-fold higher than reported previously. As isolated, the oxidized form of our preparations of the enzyme showed absorption maxima in the visible region at 460, 593 and 770 nm. The ratio of the absorption bands at 460 nm and 593 nm resulted in this protein having a strong blue colour, in contrast with the green colour of other purified copper-containing nitrite reductases. We conclude that, in contrast with previous reports, this 'blue' nitrite reductase requires both type 1 and type 2 copper centres for optimal activity.