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A thermostable purine nucleoside phosphorylase has been purified more than 800-fold from Bacillus stearothermophilm JTS 859. The enzyme had a molecular weight of 68,000 consisting of 2 identical subunits (M_w, 34,000). The isoelectric point of the enzyme was 4.7. The enzyme did not contain cysteine. The optimal pH of the enzyme reaction was from 7.5 to 11.0. The Michaelis constants for inosine, guanosine, 2'-deoxyinosine, and 2'-deoxyguanosine were 0.22, 0.14, 0.20, and 0.10mM, respectively. The optimal temperature of the reaction was 80℃. The half-life of the enzyme was 16hr in 20mM potassium phosphate and 1mM inosine (pH 7.0) at 80℃, and no decrease of the enzyme activity was observed at least for the first 30hr at 70℃.