T cells were derived from the bone marrow of 8 healthy donors and fractionated, according to their receptors for the Fc fragment of IgG, into TG+ and TG- lymphocytes. These were then cocultured with autologous or allogeneic bone marrow cells in agar in the CFU-C assay. No significant suppresion of colony formation could be detected. Total T, TG+, and TG- cells were then incubated for 18 hr with PWM, washed, and cocultured with bone marrow cells. PWM-treated TG- cells showed no significant CFU-C suppressor activity, whereas PWM-treated total T and TG+ cells inhibited colony formation of both autologous and allogeneic marrow cells. The supernatant of PWM-treated total T and TG+ cells also inhibited colony formation. PWM alone enhanced colony formation. The results of this study indicate that normal T cells can be activated in vitro to become CFU-C/suppressor cells after PWM stimulation, and that this effect is mediated by T cells with the Fc receptor for IgG.