A majority of previous studies have shown the effectiveness of nitroimidazole derivatives, metronidazole and Ro-07-0582, in specifically sensitizing the hypoxic cell componenet of solid tumours to single doses of irradiation. The present study investigated the effects of nitroimadazoel Ro-07-0582 (0.75 mg/g) on the growth of MNU fibrosarcoma when administered with fractionated radiation therapy. Delay of tumour regrowth was recorded and was compared to single dose experiments using the sensitizer. The results show that fractionated radiotherapy with the sensitizer has an advantage over fractionated therapy alone (delay factor 1.58), but not over single dose therapy with the sensitizer (delay factor 2.6). This difference may be due to the sublethal repair from re-oxygenation in fractionated radiotherapy.