The protective effect of glycine against the perturbing action of chlorpromazine (CPZ) on liposome membranes was investigated by an electron spin resonance (ESR) tecnique. Analysis using a 5-doxyl stearic acid (DS) spin probe indicated that the hydrophobic region near the surface of the liposomes containing glycine was restricted in motion by the addition of CPZ. This result suggested that the spaces between hydrocarbons in this portion would be narrowed by the penetrance of cationized CPZ molecules into the polar-hydrocarbon interface of the liposomes, in which the surface region may be tightly packed by glycine. The presence of glycine caused no difference in the disordering action of CPZ in the 12-DS reporting portion, suggesting that glycine could not influence the depth of the penetrance of cationized CPZ into the hydrocarbon phase near the surface of liposomes. The perturbing action of CPZ on the 16-DS-labeled liposomes was weakened by the presence of glycine, suggesting the reduced penetrance of undissociated CPZ molecules into the center of the bilayers (lipid core). Thus, glycine presumably prevented CPZ molecules incorporated into the liposome surface from transferring into the lipid core by its packing effect. It was shown that glycine could reduce the liposome lytic action of CPZ, without decreasing its surface activating action. This observed anti-lytic effect could be explained as a result of the above-described inhibition of the penetrance of CPZ into the lipid core.