The paper is to study radioprotective effect of Interleukin 6 (IL-6) on the spleen cells of irradiated tumor-bearing mice. After 6MV X-ray irradiation (5Gy/5f/5d), the activities of total superoxide dismutase (T-SOD) in spleens of irradiated tumor bearing mice were measured by the method of xanthineoxidase. The contents of malondialdehyde (MDA) were measured by thiobarbituric acid (TBA) fluorescence; Immunohistochemical technique was performed to detect Bcl-2 protein in spleen cells; The effects of IL-6 on natural killing activity of lymphocytes on K,562, cells were determined by Beckman liquid scintillation counter IL-6 showed significant radioprotective effect on the spleen cells of irradiated tumor bearing mice, including increasing the activities of T-SOD (,p,<,0.01) and reducing the contents of MDA (,p,<,0.05). The increased expression level of Bcl-2 in spleen cells was obersved in the IL-6 group (,p,<,0.01). IL-6 enhanced the natural killing activity of lymphocyte on K,562, cells. Decrease of antioxidase activities and increase of free radicals may be the main causes of peroxidation induced by irradiation and damage to cell activities. IL-6 has radioprotective effect on the spleen cells, which may be due to its function of increasing the expression of Bcl-2 and antioxidase activities, eliminating free radicals, lengthening the life span of spleen cells and promoting their functions.