Uridine diphosphate-D-galactose: inositol galactosyltransferase, the enzyme responsible for galactinol synthesis, was purified 41-fold from leaves of Cucumis sativus L. by a combination of gel filtration and ion exchange chromatography. Mn2+ concentration influenced the pH optimum of the enzyme. At low Mn2+ concentration (0.2 mM) the optimum was at pH 7.0. At high Mn2+ concentration (7.0 mM) the optimum was pH 5.5. The enzyme was not found in lysates of chloroplasts isolated from cucumber leaves.