In the present study the range of preirradiated doses (D,1,) and challenge doses (D,2,) for the induction of immunological adaptive response and the optimum time intervals between D,1, and D,2, were investigated in Kunming mice. The results were as follows. 1. Single whole body preirradiation by X-rays with D,1, doses of 25~100 mGy (12.5 mGy/min) could induce adaptive response of spontaneous incorporation of ,3,H-TdR into thymocytes and the reaction of splenocytes to LPS. 2. With D,2, doses of 1.0 to 1.5 Gy, the adaptive response of spontaneous incorporation of ,3,H-TdR into thymocytes and the reaction of splenocytes to ConA and LPS could be induced. 3. The optimum interval for the induction of immunological adaptive response be tween D,1, and D,2, could be 6~12 h.