The effect of complete Freund's adjuvant (CFA) on distinct T cell functions was investigated. Adjuvant was found to suppres the generation of cytolytic T cells in vivo when mixed with allogeneic P815 cells before immunization of C57BL/6 mice. Inoculation of the mice with either adjuvant or adjuvant emulsified with allogeneic cells resulted in whole splenic populations or immunoabsorbent-purified T cells that did not generate cytolytic activity in vitro against allogeneic cells. Mixing T cells from normal and adjuvant-treated mice before in vitro sensitization resulted in suppression of lytic activity. However, memory T cells were not subject to the same suppressive regulation as were precytotoxic T cells since adjuvant had no effect on subsequent boosting of memory. Adjuvant augmented the humoral response to allogeneic cells since immunization of mice with P815 cells in adjuvant resulted in prolonged cytotoxic antibody levels. This response was predominantly IgG, reflecting the enhanced activity of another T cell subpopulation, helper T cells. We conclude that the effect of adjuvant is compartmentalized among T cell subpopulations. Activity of precytotoxic T cells is diminished by suppressor cells, whereas helper T cell function is augmented. Memory T cells do not appear to be under the same regulatory control as precytotoxic cells since adjuvant does not suppress the boosting of memory T cell responses in vitro.