Gnotobiotic rats injected in the submandibular region with killed, whole Streptococcus mutans cells developed salivary antibodies directed to this microorganism. Increased levels of salivary IgA and inhibition and augmentation of agglutinin titers with anti-rat alpha-antiglobulin suggested that these antibodies were of the immunoglobulin A class. Furthermore, the rats monoinfected and immunized with homologous organisms always had lower mean caries scores than monoinfected, non-immunized rats. This reduction was evident in carious lesions on the buccal surfaces of molars and in those in sulcal areas. These results suggest that local immunization with whole S. mutans cells stimulates a specific salivary IgA response protective against caries resulting from S. mutans infection.