The delay in regrowth to 10 mm diameter of a transplanted carcinoma in mice was used to estimate the effect of the hypoxic-cell radiosensitizer Ro-07-0582. When 1 mg/g body wt. was given before a single dose of X-rays, a dose-enhancement ratio of 2-0 was found. When the drug was given immediately after irradiation, a large cytotoxic effect was observed, equivalent to an enhancement ratio of 1-3. These results were confirmed by determining the X-ray doses required for the local control of 50% of the tumours at 80 days after irradiation. The capacity of the tumour for repair of sublethal X-ray injury within 24 h was similar to that for several normal tissues.