The radiosensitivities of artificial and natural liposomes were investigated to clarify the mechanism of damage to biomembranes. Both liposomes were highly resistant to X-rays, SH-blocking agents, radiation protective agents, and these combinations in terms of leakage of K+ or glucose. The results mean that the membrane structure of lipids is resistant to these and confirm that damage to the membrane protein may be responsible for the change in the permeability of irradiated red blood cells.