Protein antigens I, I/II, II, and III were prepared from Streptococcus mutans (serotype c). Their immunogenicities and protective effects against dental caries were investigated in 40 rhesus monkeys kept entirely on a human-type diet, containing about 15% sucrose. Antigens I, I/II and, to a lesser extent, antigen II induced significant reductions in dental caries, as compared with sham-immunized monkeys. This was achieved with 1 or 2 doses of antigen, the first of which was administered with adjuvant (Freund incomplete adjuvant or aluminum hydroxide). There was no reduction in caries in monkeys immunized with antigen III. The reduction in caries in the animals immunized with antigens I or I/II was comparable to that in monkeys immunized with whole cells. Protection against caries was associated predominantly with serum and gingival crevicular fluid immunoglobulin G antibodies, which appeared to be directed against the antigen I determinant, but antibodies to antigen II, though not to antigen III, were also protective.