A streptococcal antigen (SA) of 185 kDa was isolated from Streptococcus mutans and this antigen induced in vitro helper, suppressor and contrasuppressor activities with primate peripheral blood lymphocytes. The 185-kDa SA was then treated by sodium dodecyl sulfate and yielded a 4-kDa SA which was capable of eliciting only helper activity. We have now cleaved the 185-kDa SA with cyanogen bromide, in an attempt to identify suppressor and contrasuppressor determinants. A 14-kDa SA was separated from the cyanogen bromide digest and its ability to elicit T cell and B cell functional activities was tested in rhesus monkeys. Whereas the 185-kDa SA (and 4-kDa SA) elicited high serum anti-SA antibodies and the CD4 cells showed an increase in DNA synthesis, this was not demonstrable with the 14-kDa SA. However, the 14-kDa SA, unlike the 185-kDa SA, activated a significant proportion of CD4 and CD8 cells to bind the Vicia villosa lectin (VV) and this is a characteristic feature of contrasuppressor cells. We then studied the effect of sequential immunization of monkeys with the 14-kDa SA, followed by the 185-kDa SA. The results of this showed suppression of the CD4 proliferative response, in the presence of a normal antibody production. We suggest that the split tolerance between the T cell proliferative and B cell differentiating functions might be interpreted on the basis of suppressor CD8 cells inhibiting the CD4 proliferative phase and the VV-adherent CD8 cells contrasuppressing B cell antibody formation.