The kinetics and mechanisms of suppression of the PWM-induced PFC response of human PB lymphocytes by Con A-activated suppressor cells were investigated. It was necessary that Con A suppressor cells be present early in the preocess of activation of human B cells toward antibody synthesis, but maximal suppression of the PFC response occurred later in the culture period. In addition, Con A-activated cells, although suppressing the PFC response to PWM greater than 90% of control, did not significantly suppress the blastogenic response to PWM after 3 or 5 days in culture. On the contrary, after 3 days in culture, background tritiated thymidine incorporation as well as tritiated thymidine incorporation to PWM stimulation was increased when Con A suppressor cells are added to fresh autologous peripheral blood lymphocytes. This increased blastogenic response after three days most likely represented an autologous mixed lymphocyte reaction (MLR) of Con A suppressor cells against fresh autologous non-T cells. The induction of autoreactive cells may be one of several modes of suppression of PFC responses by Con A activated suppressor cells.