The effect of purified streptococcal pyrogenic exotoxins (SPE) on the antibody response to sheep erythrocytes was studied in cultures of mouse spleen cells. Purified SPE types A, B, and C shared the ability to suppress the day 4 direct plaque-forming cell response when added to cultures. SPE A and C were most suppressive at concentrations of 0.1 to 1 ng per culture, while SPE B was active at 1 microgram per culture. Pretreatment of mice with SPE A, 3 h before removal of their spleens for culture, also produced suppression. Cell populations were separated from spleens of normal and toxin-treated mice and recombined in culture to test the cellular site of action of SPE immunosuppression. When nonadherent cells (lymphocytes) and adherent cells (macrophages) from control and SPE-treated mice were separated and recombined, the plaque-forming cell response depended on the source of lymphocytes. Macrophages from toxin-treated mice functioned normally in the presence of control lymphocytes. In a further experiment, toxin pretreatment failed to suppress the plaque-forming cell response of spleen cells that were T-cell depleted and reconstituted with control thymocytes. When the T lymphocytes were removed from toxin-treated spleen cell suspensions, the remaining cells were able to respond normally to antigen if normal helper T cells were provided. The results suggest that the suppressive activity of SPE on antibody production is mediated by altered activity of T lymphocytes.