Streptococcus mutans, the principal etiologic agent of dental caries in humans, possesses a variety of virulence traits that enable it to establish itself in the oral cavity and initiate disease. A 185-kDa cell surface- localized protein known variously as antigen I/II, antigen B, PAc, and P1 has been postulated to be a virulence factor in S. mutans. We showed previously that P1 expression is necessary for in vitro adherence of S. mutans to salivary agglutinin-coated hydroxyapatite as well as for fluid-phase aggregation. Since adherence of the organism is a necessary first step toward colonization of the tooth surface, we sought to determine what effect deletion of the gene for P1, spaP, has on the colonization and subsequent cariogenicity of this organism in vivo. Germ-free Fischer rats fed a diet containing 5% sucrose were infected with either S. mutans NG8 or an NG8- derived spaP mutant strain, PC3370, which had been constructed by allelic exchange mutagenesis. At 1-week intervals for 6 weeks after infection, total organisms recovered from mandibles were enumerated. At week 6, caries lesions also were scored. A significantly lower number of enamel and dentinal carious lesions was observed for the mutant-infected rats, although there was no difference between parent and mutant in the number of organisms recovered from teeth through 6 weeks postinfection. Coinfection of animals with both parent and mutant strains resulted in an increasing predominance of the mutant strain being recovered over time, suggesting that P1 is not a necessary prerequisite for colonization. These data do, however, suggest a role for P1 in the virulence of S. mutans, as reflected by a decrease in the cariogenicity of bacteria lacking this surface protein.