Radiosensitizing effect of sodium glycididazole (SGDD) on the hypoxic V,79, cells by standard in vitro colon formation method has been further studied. The results showed its toxicity was low. Its ID,50, in cells under hypoxic and aerobic condition were 23.5 and 35.7 mmol/L respectively. These indicated that SGDD showed more toxicity under hypoxic than that aerobic condition (,p,<,0.01). SGDD did not produce any radiosensitizing effect under air condition, however, irradiation under hypoxic SGDD showed radiosensitization and its SER increased as a function of drug concentration below 1.38 mmol/L. Its C,1.6, was 0.48 mmol/L. Its maxium SER was 2.3 at 1.38 mmol/L. Comparison of radiosensitizing effect of SGDD versus MISO and its mother compound (metronidazole) under the same experimental condition, SER for SGDD, MISO and metronidazole were 1.75, 1.53 and 1.07 at 0.3 mmol/L respectively. SGDD showed more radiosensitizing efficiency than MISO and much greater than metronidazole. This study further confirms our previous results i.e. SGDD is a hypoxic radiosensi tizer with low toxic, high efficiency and selectively enhances the radiosensitivity of hypoxic cells for tumor radiotherapy.