The proliferative capacity of mouse T cells was inhibited by radiation doses of 5 Gy or greater. (A) Murine splenocytes were irradiated with a single dose between 0 and 20 Gy and then cultured in the presence of immobilized anti-CD3 monoclonal antibody in RPMI media containing 50 U/mL IL-2 and 10% vol/vol FBS. The numbers of viable T cells were counted after 2 to 15 days of culture. (B) Groups of 2 to 5 SCID mice (H2^d) received intravenous injections of 15 × 10⁶ splenocytes prepared from a CD45.1⁺C57.BL6 (H2^b) donor and irradiated at the doses shown. One, 2, 5, and 7 days after injection, 200 μL peripheral blood were analyzed for the presence of donor-derived T cells by FACS using antibodies to CD45.1, CD3, and propidium iodide 1 μg/mL to exclude nonviable cells. The mean (n = 2 to 4) percentage of donor-derived CD3⁺, CD45.1⁺ cells among the nucleated blood cells at each time point is shown. *, Some of the mice that received nonirradiated allogeneic splenocytes were killed because of the development of GVHD at day 5 or day 7 postsplenocyte infusion.