The hemolytic action of benzyl alcohol and its possible effect on human erythrocyte membrane have been studied. The alcohol showed a severe hemolytic action above 90-100 mM. The cells exposed to the alcohol at prelytic concentrations showed increased osmotic and heat fragility. Benzyl alcohol treatment induced a slight dissolution of the components ; phospholipids were dissolved relatively more than proteins and cholesterol. Spectrin was partly released from the membrane during incubation with the alcohol above 110 mM. Use of the spin labeling technique showed that the lipid chains became much more mobile as a result of the alcohol treatment ; the apparent rotational correlation time, t_c, for probes I (12, 3) and I (1, 14) and the order parameter for I (12, 3) gradually decreased with increasing alcohol concentrations. Glucose-6-phosphate dehydrogenase activity was strongly and non-competitively inhibited by the alcohol, probably due to the extensive elution of this enzyme from the membrane. Reduced nicotinamide adenine dinucleotide (NADH) : (acceptor) oxidoreductase and Na⁺, K^- and Mg²⁺-adenosine triphosphatase activities were also significantly lowered above 100-110 mM, while acetylcholine esterase activity was not significatnly inhibited. These results indicate that the alcohol above 100 mM strongly perturbs the lipid environment and membrane structure, due to greatly increased fluidity of the lipid bilayer and disordering of the lipid-protein interactions.