The g antigen of Streptococcus sobrinus 6715 was previously shown to consist of polysaccharides of various molecular weights. In this study, two such polysaccharides, LII and LIII, were purified by gel filtration and affinity chromatography procedures. By a double immunodiffusion analysis, fraction LII was found to contain a region in the serotype-specific g site not present in the serospecific g site of fraction LIII. This region was designated x. In addition to the serotype-specific g site, the cross-reactive sites, g-a, g-d, and g-(a-d), were all present on a single molecule of fractions LII and LIII. Polysaccharides LII and LIII were composed of galactose, glucose, and rhamnose. Analysis of inhibition of the precipitin reaction suggested that the serotype g site of fraction LII (the putative form of g antigen) may consist of two immunodominant regions, one a galactose-containing region (region x) and the other a glucose-containing region, while the serotype g site of fraction LIII appeared to have one immunodominant region containing a glucose determinant. The methylation and 13C nuclear magnetic resonance analyses of LII and LIII fractions provided information on the linkage and the anomeric structures of the sugar components of the polysaccharides.