Anti-tumor activities of ACNU and X-irradiation on methylcholanthrene induced glioma in C57BL mice were studied in vitro and in vivo. In vitro experiments using cultured glioma cells (MGB cells), the synchronization of cell cycle was done by excess addition of thymidine, and the anti-tumor cell effect were investigated by mean of determinations of DNA synthesis, mitotic index and the number of the living cells following the treatments. As the results, it appeared obvious that ACNU was most effective on MGB cells in S phase and X-irradiation in M phase. As to the combined therapy of ACNU and X-irradiation, the anti-tumor effect was most remarkable when the cells were treated by X-irradiation in the G2, M phase, which were hervested by addition of ACNU 44 hours before irradiation. However simultaneous treatment of ACNU and X-irradiation on the cells in G1 phase was not so remarkable. In vivo experiments the anti-tumor effect of ACNU and X-irradiation on subcutaneously or intracranially transplanted glioma in mice was investigated. Either ACNU 10 mg/kg or local X-irradiation 1240 rads showed inhibitory effect on the tumor growth and prolonged the survival time of the tumor bearing mice. The combination therapy was more effective than ACNU or X-irradiation alone, particularly combination therapy of ACNU and repeated small doses irradiation of X-ray was remarkably effective. Evidence obtained indicates that the combination therapy of ACNU and X-irradiation have synnergistic anti-tumor effect on experimental mouse glioma.