The effect of γ-irradiation on membranes of leaf vacuoles from Chelidonium majus was investigated. Exposure of the isolated membranes to 200 Gy γ-irradiation decreased the amount of radiosensitive unsaturated fatty acids; these reductions were alleviated in vacuolar membranes irradiated in vivo. A similar alleviation was noted for SDS polypeptides. As a result of γ-irradiation on leaves, the amounts of low molecular weight substances detectable by HPLC and capillary electrophoresis were augmented in the vacuole, probably due to an alteration of permeability properties. These observations support the postulate that vacuolar membranes were damaged by the ionizing radiation and that certain mechanism(s) in normal cells protected the tonoplast from the radiation.