A multivalent vaccine consisting of whole cell antigens of seven strains, representing four serotypes (b, c, d, and g), of mutans streptotocci was used to hyperimmunize a group of cows. Serum samples from these animals contained immunoglobulin G1 (IgG1) antibody activity to seven serotypes (a to g) of mutans streptococci. Whey obtained from the animal with the highest serum antibody activity, which also contained high levels of IgG1 antibody, was used in passive caries immunity studies. Gnotobiotic rats monoinfected with Streptococcus mutans MT8148 serotype c or Streptococcus sobrinus OMZ176 (d) or 6715 (g) and provided a caries-promoting diet containing immune whey had lower plaque scores, numbers of streptococci in plaque, and degree of caries activity than similarly infected animals given a diet containing control whey obtained from nonimmunized cows. To establish the nature of the protective component(s) present in the immune whey, an ultrafiltrate fraction of the whey was prepared. This preparation contained higher levels of IgG1 anti-S. mutans antibody activity than the immune whey. Rats monoinfected with S. mutans MT8148 and provided with a diet supplemented with 0.1% of this fraction exhibited a degree of caries protection similar to that seen in animals provided a diet containing 100% immune whey. In fact, a diet containing as little as 0.01% of the ultrafiltrate fraction gave some degree of protection against oral S. mutans infection. The active component in the immune whey was the IgG1 anti-S. mutans antibody, since rats monoinfected with S. mutans MT8148 and provided a diet supplemented with purified immune whey IgG1 had significantly reduced plaque scores, numbers of S. mutans in plaque, and caries activity compared with control animals. Prior adsorption of the IgG fraction with killed S. mutans MT8148 whole cells removed antibody activity and abrogated caries protection.