Leakage of potassium from mouse fibroblast LM cells, X-irradiated at 0°C with doses up to 400 Gy is shown to be related to plasma membrane lipid composition. Fatty acid supplemented cells, containing about 40 per cent polyunsaturated fatty acids (PUFA) in their membranes were much more sensitive to radiation, as measured by increased permeability, than normal cells, which contained 7 per cent PUFA. The damage observed after irradiation at 0°C was partially repaired during a post-irradiation incubation at 22°C. The o.e.r. for potassium leakage was about 4 for normal fibroblasts and 8 for the PUFA-supplemented cells. No oxygen-dependent radiation damage could be observed in cells treated with high amounts of vitamin E. Depletion of glutathione in PUFA cells sensitized oxic cells to radiation damage, resulting in an increase of the o.e.r. from 8 to 17. No lipid peroxidation (malondialdehyde production and disappearance of fatty acyl chains) could be demonstrated. While PUFA, normal and vitamin E grown cells showed a differential sensitivity in radiation-induced potassium leakage and trypan blue uptake (high doses, interphase death), no difference in radiation-induced clonogenic ability (reproductive death) could be observed after the different cell treatments. The experiments reported are supportive of a role of membranes in the mechanism of radiation-induced interphase death and show that increased damage may be expected when high amounts of polyunsaturated membrane lipids are present under conditions of low amounts of appropriate antioxidants.