The survival of cells exposed at a high or a low dose rate to 60 Co γ-radiation, under well-oxygenated and hypoxic conditions, is compared for three strains of Chlamydomonas reinhardi differing in repair activity. In oxygen a dose-rate effect is found in all strains, but in hypoxia the mitigating effect of protracted irradiation is observed only in those strains having a certain type of repair activity. Differences in dose-rate and dose-fractionation effects observed under hypoxic conditions in mammalian and plant cells are considered in relation to possible differential repair activity.