When rats from dams fed a low protein diet were injected with whole, killed Streptococcus mutans 6715 cells in the region of the submandibular gland, they developed serum and salivary agglutinins to this microorganism. Titers of agglutinins in malnourished rats were similar to those observed in rats from dams fed a nutritionally adequate diet that were locally injected with S. mutans. Furthermore, both groups of immunized rats subsequently infected with cariogenic S. mutans 6715 had lower mean caries scores than infected, nonimmunized rats. This reduced incidence of caries was evident on all molar surfaces. The mean body weights of immunized and nonimmunized protein deficient rats were not significantly different; likewise, both immunized and nonimmunized normally nourished rats exhibited similar weight gains. Malnourished rats, not immunized but infected with S. mutans, had significantly more caries than normal, nonimmunized infected rats. Both dietary groups of noninfected rats had very few carious lesions. These results suggest that carious lesions observed in these rats resulted from S. mutans 6715 infection. Furthermore, proteinmalnourished protein malnourished rats, injected in the region of the submandibular gland with whole, killed S. mutans elicit an immune response and are protected against S. mutans induced caries.