Three inhibitors of mammalian cell respiration, rotenone, antimycin A and oligomycin, were tested for their effects on O2 utilization and radiation response of [Chinese hamster V79] multicell spheroids. All 3 agents rapidly inhibited O2 utilization of peripheral cells of intact spheroids, leading to a progressive increase in radiosensitivity as O2 reached the centrally located cells. Virtually complete reoxygenation of the spheroid cells was observed with only 0.12 .mu.M rotenone; a similar radiation response with antimycin A or oligomycin required concentrations of 10 and 250 .mu.M, respectively.