Inhibition of repair of sublethal and potentially lethal damage was observed in respiratory-deficient mutants of yeast during fractionated X-irradiation in the presence of equimolar concentrations of 2-deoxy-D-glucose and glucose in the growth medium. In the wild-type cells, on the other hand, an enhancement of repair of the potentially lethal damage was obtained under similar conditions. These results suggest, by analogy, that in higher organisms also, 2-deoxy-D-glucose may differentially inhibit the repair of radiation damage in hypoxic tumor cells while enhancement of repair processes could be expected in normal tissues.