1. When D-erythrose was incubated with beef liver preparation, a distinct decrease in reducing power, measured by Willstatter-Schudel's method was observed.2. The enzyme has been prepared in a soluble form from beef liver and purified about thirty fold.3. The enzyme shows pH optima at 6.0 and 7.5. During the reaction little oxygen uptake, little carbon dioxide output and litle change of optical density at 340mμ were observed. The enzyme acts more effectively upon D-threose than upon D-erythrose, but has no activity on the following compounds: D-ribose, D-xylose, D-arabinose, D-lyxose, D-glucose, D-galactose, D-mannose.4. From the reaction mixture containing D-erythrose as the substrate, erythritol and D-erythronic acid were detected by paper chromatography. When D-threose was used as the substrate, the reaction products were D-threitol and D-threonic acid.5. By treatment with alumina Cγ gel, the partially purified preparation was fractionated into a supernatant component (A) and an adsorbable, sodium citrate-extractable component (B). A or B alone showed little or no activity. When these components were mixed, almost the full activity was recovered.6. When B was mixed with catalytic amounts of NAD, the activity was largely regained. NAD could not be replaced by NADP. A as well as B is heat labile and non-dialyzable. Therefore, A seems to be the bound form of NAD-like substance.7. On the basis of these results, it can be expected that dismutation of D-tetrose to the corresponding polyol and acid in the presence of NAD-like substance takes place by the action of liver enzyme.