The serotype antigens of Streptococcus mutans have been described as cell wall-associated polysaccharides. In this study, an additional wall polysaccharide antigen was purified and characterized from S. mutans strain 6715-T2, a mutant of 6715 (serotype g). Strain 6715-T2 lost the serotype antigen during animal passage. Rhamnose-containing carbohydrate fractions were solubilized from bacterial cells by extraction with 5% trichloroacetic acid at 4 degrees C for 18 h and with 0.01 N HCl at 100 degrees C for 20 min. Extracts were combined and purified on columns of diethylaminoethyl-Sephadex A-25 and Sephadex G-100. The purified sample contained 59% rhamnose, 31% glucose, 2.2% protein, and 0.24% phosphorus. The purified rhamnose-glucose polysaccharide (RGP/6715-T2) reacted strongly with antisera to whole cells of 6715-T2. Agar gel diffusion and comparative immunoelectrophoresis studies revealed that RGP/6715-T2 was serologically distinct from the serotype g and d polysaccharide antigens. These techniques also indicated immunological identity between RGP/6715-T2 and RGP/B13, a rhamnose-glucose polymer previously isolated from S. mutans B13, a serotype d strain. Antigen immunologically identical to RGP/6715-T2 was detected both in Rantz-Randall extracts from whole cells of S. mutans strains 6715, OMZ-65, and 6715-PT and in extracts from cells of 6715-T2 and C307, two mutant serotype g strains that lacked the serotype g antigen.