alpha-Glucosyltransferase was purified from Serratia plymuthica ATCC 15928. This enzyme was designated as a sucrose isomerase. its molecular weight was estimated at 65,000, the isoelectric point was 9.0, and the specific activity was 120 IU mg(-1). The K-m for sucrose was estimated at 65 mM. Glucose was demonstrated to be a competitive inhibitor. The purified sucrose isomerase was found to mediate sucrose conversion into several products: isomaltulose, trehalulose, isomaltose, glucose, fructose, and isomelezitose. The maximum conversion was obtained at pH 6.2 and 30 degrees C, but the product distribution was shown to depend on the reaction temperature. Addition of glucose or fructose to the reaction mixture also modified this distribution. These observations allowed us to propose an explanation by a kinetic scheme combining an intra-and an intermolecular mechanism.