Antigen-specific suppressor factor could be released from monkey suppressor T cells induced in vitro with a protein antigen isolated from the carcinogenic bacterium Streptococcus mutans. The suppressor activity was due to the factor itself and not to carryover of free antigen. Characterization of the monkey factor revealed it to have a molecular weight of ca. 70,000, and to contain a constant region and determinants encoded by the major histocompatibility complex. The presence of immunoglobulin determinants could not be demonstrated. However, by virtue of its adsorption to specific antigen, an antigen-combining site was shown to be present. The possible regulatory role of streptococcal antigen-specific suppressor factor in protection against dental caries is discussed.