Spleen cells from chickens injected with sheep erythrocytes (SE) intravenously 2 to 14 days prior to culture were found to give faster and higher plaque-forming cell responses upon addition of antigen on day 2 rather than on day 0 of culture. Cell mixture experiments showed that this was due to the induction of suppressor T cells upon re-exposure to SE on day 0 of culture. Spleen cells taken on days 2 or 14, but not between days 4 and 7 after priming to SE were sensitive to suppression. The suppressor cells were resistant to gamma irradiation (1000 rd) and to mitomycin C, but were apparently lost after 2 days of culture in the absence of antigen. Pokeweed mitogen addition on day 0 of culture also induced suppressor cells, both in SE immune and in normal spleen. Similar suppressor cells were induced in cultures of primed spleen cells taken from agammaglobulinemic chickens. The response to Brucella abortus in vitro was not affected by induction of suppression for the anti-SE response. Suppression could also be shown after transfer of cell mixtures to irradiated recipients. Helper cell activity for the anti-SE response could readily be shown, both in vivo and in vitro, in primed spleen cells precultured for 2 days in the absence of antigen, and was also resistant to 1000 rd gamma irradiation and to mitomycin C.