Primed and unprimed murine spleen cells and suspensions enriched for T or B cells were exposed to various doses of radiation in vitro and evaluated with respect to their capacity to respond to sheep red blood cells (SRBC) in 1-ml or 10-microliter cultures. The dose-response data show three components: (1) a low dose (less than 50 rad) phase associated with an augmented anti-SRBC response; (2) a radiosensitive phase associated with precipitous loss of anti-SRBC activity; (3) a relative radioresistant phase with a loss of anti-SRBC activity which occurs over a broad range of radiation doses. Data obtained utilizing irradiated subpopulations enriched for T or B cells combined with excess numbers of nonirradiated lymphocytes of the corresponding cell type suggest that low dose augmentation is the result of radiation-induced injury to T cells or subcomponent thereof. With respect to the second component of the dose-response curve, similar experiments show that B cells are more radiosensitive than T cells (D37 of approximately 80 vs. approximately 220 rad, respectively) in this response. The third component of the curve appears to involve the interaction of radio-resistant subpopulations of T and B cells.