This study deals with the comparative effects of gamma irradiation on T and B lymphocytes in mice. Quantitatively, splenic T cells appeared substantially more radioresistant than B cells. However, in the thymus, the mesenteric lymph node and peripheral blood, both cell types were highly radiosensitive. No repair processes could be detected with respect to lymphocyte killing, since reducing the dose rate or fractionating the dose resulted in no reduction in cell destruction. Splenic B cells which remained intact 3 days after doses of 600 R or larger were impaired in their ability to form caps, whereas T cells capped normally. Spleen cells which were viable 3 days after irradiation bore latent radiation damage which was manifested as a reduced ability to survive and generate plaque-forming cells in vitro. Attempts to separately assess irradiated T and B cell immunocompetence in vitro suggested that at doses of 300 R and below, surviving B cells were more impaired than T cells. After a 600 R dose, neither cell type appeared to be functional.