Dextran-glucosidase EC 3.2.1.70 isolated from Streptococcus mitis ATCC 903 was tested for activity on water-soluble polysaccharides produced from sucrose by four Streptococcus mutans strains: AHT, BHT, IB and B13. Two fractions of glucans with molecular weights of about 40,000 and < 9,400 were obtained by gel filtration of water-soluble glucans of S. mutans. The elution profiles were essentially the same for the four strains. Upon incubation of these glucans with dextran-glucosidase, only 10–30% of the low-molecular weight glucans remained. The high-molecular weight glucans were resistant to dextran-glucosidase or were degraded to 25%. Glucose was the degradation product. The yields of water-soluble polysaccharides from the S. mutans strains varied between 17.6 mg/ml culture supernatant (IB) to 1.4 mg/ml (B13