X-radiation decreases the number of RBC in vivo and accelerates hemolysis in vitro, but the mechanism is still unclear. Taking the view that these phenomena are due to radical formation and active enzyme formation followed by accelerated peroxidation of cell membrane lipid and membrane structural changes we studied the morphological changes of RBC, changes in ion-partition, and the inhibitory effects of drugs. The results were as follows: 1) Hemolysis of human erythrocytes (RBC) was accelerated by x-radiation. The degree of such lysis was proportional to the degree of x-radiation. This lysis was inhibited markedly by the addition of cephalanthin. The inhibitory effect was dependent on the concentration. In high concentrations conversely acceleration occurred. 2) Soon after x-radiation and prior to hemolysis, human RBCs released K(+) bringing about a decrease in the K(+) partition. The time of this decrease paralleled the degree of x-radiation. The K(+)-partition decrease caused by x-radiation was inhibited by the addition of cephalanthin. 3) In parallel with the acceleration of erythrocyte membrane permeability by x-radiation, changes in RBC morphology were more likely, thus causing structural changes in the cell membrane. 4) Cephalanthin protects the RBC from its instability to x-radiation, and at certain concentrations it restores normal morphology to RBC that have an Echinocyte structure, but at high concentrations it makes RBC to assume a stomatocyte shape. 5) The above findings are discussed in terms of cell membrane injury mechanisms and the inhibitory effect of cephalanthin.