Because of the unsuccessful clinical trials of misonidazole (MISO), many efforts have been made to find a new hypoxic cell sensitizer which is more effective and/or less toxic than MISO. In Japan, RK-28 is already under going Phase II clinical evaluation and RP-170, KU-2285 and KIH-802 have been proven to be effective both in vitro and in vivo and are also waiting further clinical trials. But in U.S.A. and England, etanidazole (SR-2508) and pimonidazole (Ro 03-8799) are under going Phase III evaluation and clinical evaluation of SR-2508 will be key opened in early next year. Now, an appropriate goal for a new radiosensitizer would be more effective and/or less toxic than SR-2508 or clinically more useful than SR-2508. Clinical trials of SR-2508 have been performed using intravenous injection, however it may be difficult to give a sensitizer intravenously on a daily basis in accordance with requirements for standard fractionated radiotherapy in a clinical setting. In contrast, RP-170 and KU-2285 have the potential to produce considerable radiosensitization under both intravenous and oral administration.