The effect of 5-thio-D-glucose on the viability and radiosensitivity of P815-X2 mouse mastocytoma cells was studied in vitro. By itself, this compound is specifically cytotoxic toward hypoxic cells. It sensitizes hypoxic cells to irradiation while it protects aerobic cells from radiation damage. These features, together with its low toxicity, suggest that 5-thio-D-glucose may be a useful adjuvant to radiotherapy.