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The effect of endotoxin fromSalmonella typhimurium orEscherichia coli was studied on bilayer lipid membranes (1% lecithin-1% cholesterol in n-decane) formed in buffered 0.1M NaCl solution (pH 6.8). Endotoxin was added to the buffered solution either prior to membrane formation or after stable membranes were formed. In both cases, concentrations of 110 to 720 μg/ml endotoxin initiated a decrease in the electrical resistance of the membranes followed by their rupture. A 50 μg/ml concentration of the agent was unable to elicit any response. Also, the addition of an equal volume of buffer solution, serving as a control, caused no decrease in membrane resistance or survival time. Treatment of the endotoxin with alkaline hydroxylamine to remove esterand amide-bound fatty acids likewise abolished the membrane effect. This is the first report of an endotoxin effect on lipid bilayer membranes. The potential correlation of this interaction of bilayer and endotoxin with the diverse biologic effects of endotoxin is discussed.