The serotype polysaccharide g from Streptococcus mutans 6715 was found to cross-react with serotype polysaccharide a from S. mutans HS6 and serotype polysaccharide d from S. mutans B13. Double immunodiffusion experiments indicated that the serotype polysaccharide g consisted of the following: (i) the type-specific g site; (ii) a cross-reactive site g-a that was in common with polysaccharide a; (iii) a cross-reactive site g-d that was in common with polysaccharide d; and (iv) a cross-reactive site g-(a-d) that was in common with both polysaccharides a and d. Moreover, by a procedure involving several column chromatography steps, six polysaccharide-containing fractions showing reactivity with anti-g serum were found. By gel filtration, the molecular weight estimates of fractions LI, LII, LIII, SI, SII, and SIII were 2 X 10(6), 5 X 10(5), 6 X 10(4), 3 X 10(4), 1.4 X 10(4), and 1 X 10(4), respectively. Double immunodiffusion analysis indicated that LI, LII, and LIII contained the four antigenic sites of the putative polysaccharide g. LII also contained another additional immunodominant region, designated site x. The analysis also suggested that fraction SI lacked the type-specific site g, fraction SII lacked sites g and g-a, and fraction SIII lacked sites g, g-a, and g-d.