An investigation into occurrence of dextranase in human oral streptococci revealed that cell extracts of 8 out of 25 strains contained an enzyme, α-1,6-glucan glucohydrolase, that hydrolyzed the α-(1 → 6)-glucosidic linkages in dextran and isomaltose. Glucose was the only product of low molecular weight released by this enzyme. Six strains contained a different dextranase that released oligosaccharides of the isomaltose series from dextran. In the case of Streptococcus mutans IB16, it was established that the main oligosaccharide products were isomaltotetraose and isomaltopentaose. Cell-free filtrates from 14 of the 25 strains synthesized extracellular polysaccharides from sucrose, and eleven of the filtrates produced water-insoluble glucans. Dextranase occurred in five of the strains that synthesized glucan and in five strains that were unable to synthesize extracellular polysaccharides. All the insoluble glucans were partially hydrolyzed by a fungal α-(1 → 3)-glucanase. None of the cell-extracts or cell-free filtrates was able to hydrolyze α-1,3-glucan.