[en] The significance, present status of study and problems of hypoxic cell sensitizer in radiotherapy were examined especially on misonidazole. This drug intensified the radiosensitivity of only the tumors containing hypoxic cells without affecting normal cells. It spreads and permeates from blood vessels to hypoxic cells in the tumor, and the intratumoral concentration is 50 - 80% of the blood concentration. Animal studies in Japan demonstrated that the permeability of this drug into tumors was 70 - 80% of the blood level, and that TCD50 was significantly decreased, producing sensitization rates of 1.2 - 1.3 with 0.1 mg/g and approx. 2.0 with 1.0 mg/g. However, data on the sensitizing effect of fractionated irradiation vary from report to report, and it is a future theme of study. As acute toxicity, the LD50 was approx. 2 mg/g for animals, and in humans, the administration of 0.5 mg/g 3 times per week for 6 weeks resulted in neurotoxicity. Its mutagenicity has been reported. In the phase 1 clinical study, the blood level 4 - 6 hours after the oral administration of this drug in 97 cases was in proportion to the dose; 1.0 g/m₂ produced sensitization rates of 1.4 - 5%, and 2.0 g/m₂ those of 1.6 - 7%. It was effective in approx. 30%, and side effects were noted in 32 cases. the toxicity of this drug to hypoxic cells has also been examined. (Chiba, N.)