SummaryThe effect of 15 Mev electrons and x-rays from a linear accelerator on the permeability of Ehrlich-Landschutz murine ascites carcinoma cell membrane has been studied in vitro by measuring the loss of potassium ions from the cells into a potassium-free medium. The cells were washed in saline (0·145 M NaCl made 10−4 M with respect to NaHCO3; pH 7·2 ± 0·2) and saline suspensions were irradiated over the dose range 20 000 to 85 000 rads in the presence of air, ‘excess’ oxygen, 2-methyl-1,4-naphthaquinol bis (disodium phosphate) [MNDP] (Synkavit-Roche Products Ltd.) (10−5 M) and 2-amino-3-methyl butane thiol [AMBT] (10−4 M). The experiments have shown that the irradiation causes a significantly greater loss of potassium from the irradiated cells compared with the control unirradiated cells, and this is more marked in the presence of MNDP and ‘excess’ oxygen. The radiosensitizing effect of oxygen, MNDP and the radioprotecting effect of AMBT are clearly demonstrated. Possible mechanisms for the radiation ...