The extracellular, water-soluble and cell-associated, 1 N NaOH-soluble glucans from cariogenic Streptococcus mutans 6715-13 "wild type" (WT) and glucan synthesis-defective mutants with diminished virulence have been quantitatively and qualitatively analyzed by methylation analysis and gel chromatography. The mutants synthesized more of a highly branched alpha-(1 --> 6)-rich extracellular polymer than WT, and some of this glucan was also found to be cell associated in all but one case. WT, in distinction to the mutants, also synthesized a highly branched, alpha-(1 --> 3)-rich, cell-associated polymer. Treatment of these two distinct polymer types with dextranase or an alpha-(1 --> 3)-hydrolyzing enzyme indicated they were composed of both alpha-(1 --> 3) and alpha-(1 --> 6) linkages and of alpha-(1 --> 6) with branches at the 3-position, rather than of separate alpha-(1 --> 3) and alpha-(1 --> 6) homopolymer mixtures. Gel chromatography before enzymatic hydrolysis disclosed a high degree of polydispersity in both glucan classes. After hydrolysis polydispersity was reduced, again without resolution of two glucan populations. These findings suggest that (i) there are two distinct glucan classes, one alpha-(1 --> 3) rich and the other alpha-(1 --> 6) rich in WT, (ii) diminution of virulence in the mutants is probably ascribable to a failure to form the alpha-(1 --> 3)-rich component, (iii) both alpha-(1 --> 6)- and alpha-(1 --> 3)-rich glucans are found in association with the cell, and (iv) both highly branched glucan types are dextranase and alpha-(1 --> 3)-hydrolase sensitive, and methylation analysis and gel chromatography suggest polymers with highly polydisperse molecular weights which contain mixtures of linkage types.