1) The selective induction of secretory antibody is antigen dose dependent. A dose of 10(7) or 10(8) S. mutans CFU (per gram of diet) elicited a significant response, while a higher dose (10(9) CFU/gram) resulted in an apparent state of unresponsiveness. 2) A lowly virulent mutant of S. mutans 6715 (C4) induced protective s-IgA antibodies against challenge with a highly virulent species (C211). Cross protection among serotypes of S. mutans has been achieved. 3) Bovine milk with antibodies to S. mutans significantly reduced the level of caries lesions in rats fed this lyophilized milk and challenged with the homologous, virulent S. mutans strains.