Glycerol dehydrogenase of high purity was isolated from Bacillus megaterium. The enzyme has a pH optimum of 9 and dehydrogenizes in presence of NAD+ glycerol as well as 1,2-propanediol and, to a smaller extent, erythritol. The Michaelis constant for glycerol is 1.4 X 10(-3)M and for NAD+ 3 X 10(-4)M. With the application of different methods (density gradient centrifugation, gel electrophoresis and gel chromatography) relative molecular masses of 156 000-160 000 were ascertained. In the dodecyl sulfate electrophoresis we found a molecular mass of 38 000 per subunit, so that the native enzyme is existent in the tetrameric form. By means of fluorescence titration one NADH binding site per subunit could be estimated. The amino-acid composition was determined. The enzyme is stable only in the presence of high thiol concentrations. Heavy metal ions and chelating agents inhibit the activity of the enzyme. Zinc, presumably, is the natural heavy metal in the active center, but other divalent metal ions can also activate the apoenzyme. With the help of chemical modification reactions, the probable presence of one essential thiol group and one essential tyrosine residue per subunit could be demonstrated.