Hot-pulse treatment of parental thymus or spleen cells increased their ability to simulate antibody synthesis in F1 hybrid recipient mice. A similar effect was obtained after treatment of the cells with low doses of X-irradiation. Likewise, irradiation of syngeneic unprimed cells made them stimulatory on antibody production. The production of IgG, IgM as well as IgE antibodies was enhanced after transfer of irradiated cells. The results suggest that irradiation preferentially inactivates suppressor cells, that are present in the thymus and the spleen.