Using Chinese hamster cells (V79), irradiated in vitro with x-rays, we plan extensive tests to re-evaluate the earlier conclusions regarding (a) the importance of hydroxyl radicals (.OH) in causing lethal damage, and (b) the specific relationship between damage from oxygen and OH radicals. The model we are testing states that oxygen functions as a radiation sensitizer, at least in part, by reacting at cellular sites formed after .OH attack.