Alternansucrase, an exocellular glucansucrase from Leuconostoc mesenteroides NRRL B-1355, which synthesizes an alternating (alpha)-(1(--->)3), (alpha)-(1(--->)6)-D-glucan from sucrose, was isolated from the culture supernatant fluid of cultures of this organism. The most effective method for accomplishing this was hydrophobic chromatography on phenoxyacetyl cellulose. Alternansucrase was shown to synthesize alternan and to form both (alpha)-(1(--->)6) and (alpha)-(1(--->)3) glucosidic bonds by acceptor reactions with low-molecular-weight saccharides in the presence of sucrose, but an (alpha)-(1(--->)3) bond was synthesized only when an (alpha)-(1(--->)6) glucosidic bond was present at the nonreducing end of the acceptor. Acceptor reactions occurred by transfer of glucosyl units from sucrose to the nonreducing ends of oligosaccharide acceptors;A mixture of two exocellular glucansucrases from L. mesenteroides NRRL B-742 was found to be capable of transferring glucosyl units from sucrose to L. mesenteroides B-512F dextran, to form (alpha)-(1(--->)3) branch linkages via acceptor reactions. It was demonstrated that only one of the two glucansucrases present in the mixture was responsible for these branching reactions; this enzyme is a dextransucrase which forms a dextran having an (alpha)-(1(--->)6) backbone chain with a high percentage of single glucosyl branches linked (alpha)-(1(--->)3) to the main chain. This percentage of branch points can vary, depending on the conditions under which the dextran is synthesized;A glucansucrase from Streptococcus mutans 6715, which produces a highly branched, water-soluble dextran, was found to be capable of forming (alpha)-(1(--->)3) branch linkages in the same manner as the B-742 S dextransucrase, i.e., by acceptor reactions with relatively unbranched dextran, such as that from L. mesenteroides B-512F. This streptococcal dextransucrase was stimulated by the addition of exogenous dextrans. The stimulation was greatest with relatively unbranched dextrans, while more highly branched dextrans were less effective in their ability to stimulate S. mutans 6715 dextransucrase. Other (alpha)-D-glucans, such as glycogen, pullulan, and alternan, did not stimulate;The dextransucrase from S. mutans 6715 was able to utilize alternate glucosyl donors, such as dextran, maltotriose, panose, and isomaltodextrins containing three or more glucose units, in what could be considered the reverse of acceptor reactions.