An extracellular adenosine deaminase was isolated from the culture supernatant of Nocardioides sp. J-326TK and purified 193-fold to homogeneity. It had a specific activity of 4677 units/mg at 37 degrees C, was a monomeric protein as judged by SDS/PAGE, and was characterized with respect to M(r) (80,000 and 72,000 by gel filtration on Sephadex G-200 and SDS/PAGE respectively), pH optimum (6.0), temperature optimum (50 degrees C) and pI (7.6). The adsorption spectrum of the enzyme had a maximum at 280 nm and a minimum at 250 nm. The enzyme was stable at pH 6.5-7.5 and at temperatures below 30 degrees C. Adenosine and 2'-deoxyadenosine were deaminated and the respective Km values were 0.22 and 0.20 mM, but the enzyme was not active on adenine and 6-(gamma gamma'-dimethylallylamino)purine riboside. The enzyme reaction was promoted by Fe3+ and Sn2+, but potently inhibited by Hg2+, Ag2+, o-phenanthroline and pentachlorophenol, and noticeably inhibited by 8-bromoadenosine, theobromine and theophylline.