Conventional outbred rats were fed Streptococcus sobrinus for 24 weeks and ELISA was used to identify isotypes of antibodies against bacteria in saliva and serum. Quantities of antibodies from experimental rats were compared with values derived from the control population. Saliva IgM and IgA anti-S. sobrinus from experimental rats were greater than controls at week 3, were much less at week 9, but normal levels were found by week 13. IgG antibodies in saliva peaked at weeks 5 and 9 but fell to control levels by week 13. Relative levels of antibodies in saliva of experimental animals continued to cycle during weeks 13-24 but did not differ greatly from controls. Serum IgM and IgG antibodies to S. sobrinus were essentially like controls throughout the experiment. Serum IgA increased briefly during the first 12 weeks then returned to normal levels. The results suggest that prolonged peroral exposure to cariogenic bacteria ultimately leads to modulation of antibody around unimmunized control levels even though antigenic stimulation persists.