Specific suppressor cells in mice and monkeys can be induced with high doses of Streptococcus mutans antigen. When these cells are further cultured with a low dose of S. mutans antigen in vitro, they secrete specific suppressor factors which decrease the cooperative responses to this antigen. In view of the sensitivity of these cells to anti-Thy 1 and complement, and as they are not retained on nylon-wool, these suppressor cells are T cells. The suppressor factor exerts its effect on T helper cells and not on B cells. Suppressor cells and factors may regulate antibody responses to S. mutans and might be of significance in determining the dose and frequency of immunization and the type of adjuvant to be used.