The hypoxic radiosensitizer misonidazole decreases survival in dense cultures of HeLa cells irradiated with gamma rays in a non-dose modifying fashion. Survival curves of treated hypoxic cells display a much larger extrapolation number than untreated cells. In oxygenated randomly dividing cells, drug treatment has an effect opposite to that in hypoxic cells, increasing survival. In cultures initiated from mitotic cells and irradiated soon afterwards, a smaller sensitization under hypoxia and no increase in survival of oxygenated cells was observed. It was concluded that metabolic as well as radiochemical events take place in misonidazole-treated and then irradiated HeLa cells which modify survival.