a b s t r a c tCholesterol plays an important role in the interaction of Alzheimer's amyloid beta (Ab) with cell membranes, an important event in Ab-induced cytotoxicity. However, it is not fully understood how cholesterol influences the association of Ab with membrane lateral compartments. We have shown that by modulating membrane fluidity, cholesterol decreased peptide localization in solidordered domains and increased that in liquid-ordered domains. It changed the amount of Ab associating with liquid-disordered (Ld) phase with different tendencies depending on the composition of heterogeneous membrane systems. 7-Ketocholesterol, an oxidized derivative of cholesterol, majorly enhanced the fluidity of and Ab interaction with Ld phase. These findings are useful for clarifying the impact of cholesterol and its oxidation in Ab-induced toxicity.