The effect of local immunization with glucosyltransferase enzymes of Streptococcus mutans on dental caries in conventional rats, hamsters, and gnotobiotic rats was studied. Injection of these animals with crude or defined glucosyltransferase enzyme preparations incorporated into complete Freund's adjuvant consistently resulted in the presence of antibody in saliva demonstrable by functional inhibition of enzymatic activity and binding of radioactive enzyme. Serum antibody was also present. Two experiments have been completed in conventional rats and one in gnotobiotic rats in which the animals were immunized with crude enzyme antigens. The immunized groups of animals always had lower mean caries scores than comparably sham-immunized or nonimmunized control groups. Hamsters immunized with a defined enzyme preparation, containing no more than three antigenic components (two of which were enzyme), also demonstrated significant reductions in mean caries scores. The numbers of lesions were also always lower in immunized animals. In some cases there were reductions in the numbers of S. mutans that could be recovered from the teeth of immunized, infected animals. The reductions in dental caries and lesions were greater on smooth dental surfaces than on occlusal surfaces, which might be explained as interference with adherence phenomena demonstrated by S. mutans. It is proposed that antibody interference affects dental caries caused by this organism.