Cholesterol-and glycosphingolipid-rich microdomains, called ''lipid rafts,'' are suggested to initiate and promote the pathophysiology of Alzheimer's disease by serving as a platform for generation, aggregation, or degradation of amyloid-b protein (Ab). However, methods for biochemical isolation of these microdomains may produce artifacts. In this study, when synthetic Ab1-40 monomers were added to the brain fragment at a final concentration of 2.1 mM, followed by homogenization and isolation of lipid rafts by an established method, Ab1-40 accumulated as oligomers in the lipid raft fraction. However, in the absence of a brain homogenate, synthetic Ab1-40 did not accumulate in the lipid raft fraction. When fractionation was performed in the absence of synthetic Ab1-40 and synthetic Ab1-40 was incubated in an aliquot of each fraction, a marked oligomerization of Ab1-40 was observed in the lipid raft aliquot. These results indicate that exogenous Ab associates with lipid rafts, and Ab bound to rafts forms oligomers during the isolation of lipid rafts. In addition, endogenous Ab1-40 in a Triton X-100-insoluble fraction of a brain homogenate of the Tg2576 transgenic mouse model of Alzheimer's disease formed oligomers when the fraction was incubated at 48C for 20 hr. Thus, one should be careful when one discusses the role of lipid rafts in amyloid precursor protein processing and in the generation, aggregation, and degradation of Ab.