PURPOSE: This study was performed to investigate the effects of immune cell activation and the antitumor effect for the combination of treatment with X-irradiation and Eleutherococcus senticosus Maxim Root (ESMR) on mouse tumor cells. MATERIALS AND METHODS: ESMR (250g) was extracted with 80% methanol, concentrated under decompression and lyophilized. To determine whether ESMR is able to activate the immune cells or not, the proliferation of splenocytes in vitro and the number of B cells and T cells in splenic lymphocytes in ESMR-pretreated mice were evaluated. X-irradiation was given to the mouse fibrosarcoma tumor cells (FSa II) by 250 kv X-irradiation machine. The cytotoxicity of ESMR was evaluated from its ability to reduce the clonogenecity of FSa II cells. In X-irradiation alone group, each 2, 4, 6 and 8 Gy was given to FSa II cells. In X-irradiation with ESMR group, 0.2 mg/ml of ESMR was exposed to FSa II cells for 1 hour before X-irradiation. RESULTS: The proliferation of cultured mouse splenocytes and thymocytes were enhanced by the addition of ESMR in vitro. The number of B cells and T cells in mouse splenic lymphocytes was significantly increased in ESMR pretreated mice in vivo. In FSa II cells that received a combination of 0.2 mg/ml of ESMR with X-irradiation exposure, the survival fraction with a dose of 2, 4 and 6 Gy was 0.39+/-0.005, 0.22+/-0.005 and 0.06+/-0.007, respectively. For FSa II cells treated with X-irradiation alone, the survival fraction with a dose of 2, 4 and 6 Gy was 0.76+/-0.02, 0.47+/-0.008 and 0.37+/-0.01. The difference in the survival fraction of the mouse FSa II cells treated with and without ESMR was statistically significant (p<0.05). CONCLUSION: Treatment with ESMR increased cell viability of mouse splenocytes in vitro and especially the subpopulation of B cells and T cells in splenocytes in ESMR-pretreated mice. However, treatment with ESMR did not increase the level of Th and Tc subpopulations in the thymocytes. Treatment with the combination of ESMR and X-irradiation was more cytotoxic to mouse tumor cells than treatment with X-irradiation alone; this finding was statistically significant.