The work is to explore the effects, and the mechanism, of melatonin (MLT) on the damage of immune functions in mice induced by ionizing radiation. MLT was given to C,57,BL / 6J Kunming mice through intraperitoneal injection 1h before whole-body irradiation with 2Gy X-rays. Twenty four hours after irradiation, the number of thymocytes and splenocytes,3,H-TdR incorporation rate (HTIR) of thymocytes and T and B lymphocyte transforming rates (TLTR and BLTR) induced by Con A and LPS were observed. The number of thymocytes and splenocytes, HTIR, TLTR and BLTR of the irradiated mice were lower significantly than those of the sham-irradiation group (,p,<,0.001). For all groups injected with 0.5—10mg•kg,-1, of MLT, the numbers of thymocytes and splenocytes were significantly higher than that in 0mg group (simple irradiation), with the 0.5mg•kg,-1, MLT group being the highest. HTIR of thymocytes also increased significantly (,p,<,0.01 or ,p,<,0.001), with the 0.5mg•kg,-1, MLT group being the highest TLTR induced by Con A in the 0.5mg•kg,-1, MLT group increased most significantly, while BLTR in 10mg•kg,-1, MLT group increased most significantly. In addition, 2.5mg•kg,-1, of MLT enhanced splenocyte LTR without the stimulation of the mitogens. It is concluded that MLT can reduce the damage to lymphocytes of the irradiated mice and has the protective effect on immune functions.