Two antigenic polysaccharides were extracted from cell walls of the cross-reactive strains Streptococcus mutans AHT (a) and S. mutans B13 (d). The antigens extracted from walls by the hot formamide method, were purified by affinity chromatography on columns containing the galactose-specific lectin from the castor bean and were found to be diheteroglycans consisting of galactose and glucose. Antigenic specificities of both the serotype-specific and the cross-reactive sites on each polymer were studied: the AHT (a) antigen is determined by D-galactose linked 1 leads to 6 to adjacent sugar, the B13 (d) antigen is determined by D-glucose similarly linked to o its neighbor, and the cross-reactive (a--d) site present on both polymers consists of D-galactose linked 1 leads to 6 to a subterminal sugar moiety. Methylation analysis revealed structural similarities between the purified polysaccharides that may reflect the nature of the cross-reactive sites and differences that may reflect the natures of the specific haptenic regions. Based on these studies, a partial hypothetical structural model is proposed.