Human erythrocytes suspended in plasma, or in phosphate buffered saline (PBS), were exposed to ionizing radiation. Potassium leakage from irradiated erythrocytes is significantly higher in PBS than in plasma. The potassium leakage decreases when PBS is gradually replaced by plasma. These findings suggest that some of the plasma constituents have radioprotective properties. The potassium leakage per cell is independent of the hematocrit, Hct. The potassium leakage is attributed to the formation of radiation defects in the membrane. Analysis of the effect of radiation dose, plasma and cell concentrations on the product of the number and surface area of the radiation defects indicates that the radiation damage is mainly due to the direct formation of free radicals in the cell membrane. The radioprotective effect of plasma is attributed to surface reactions of these free radicals with plasma constituents adsorbed on the membrane.