Representative strains of Streptococcus mutans serotypes a to g were examined for characteristic in vitro traits thought to contribute to cariogenic virulence. Most strains of serotypes a, d and g produced higher proportion of the cell-associated, alkali-soluble glucans from sucrose. Cells of these serotypes agglutinated upon addition of high-molecular-weight dextrans or sucrose, but did not adsorb well to saliva-coated hydroxyapatite. Serotype c, e and f strains, by contrast, synthesized higher proportions of the extracellular, water-soluble glucans and adsorbed well to saliva-coated hydroxyapatite particles but did not agglutinate with dextrans. Thus, serotype a, d and g S. mutans and type c, e and f S. mutans formed two separate groups on the basis of these biochemical properties. Synthesis of intracellular iodophilic polysaccharides from glucose showed a different pattern of distribution among the serotypes: types a, d, e and f synthesized large amounts of this polysaccharide, while all but one of the eight type c and g strains produced only small amounts. Serotype b S. mutans seemed to constitute a separate subgroup in terms of the above in vitro traits.