Previous studies have implicated the failure to degrade aggregated Aβ1‐42 in late endosomes or secondary lysosomes as a mechanism for the accumulation of β‐amyloid in Alzheimer's disease. We examined the consequences of intracellular accumulation of Aβ1‐42 on the integrity of the endosomal/lysosomal compartment by monitoring Lucifer Yellow fluorescence and the release of lysosomal hydrolases into the soluble, cytosolic fraction. In control cells, the Lucifer Yellow fluorescence is observed as punctate staining in a perinuclear distribution with no apparent cytoplasmic fluorescence, consistent with its localization in late endosomes or secondary lysosomes. After incubation with Aβ1‐42 for 6 hr, a loss of lysosomal membrane impermeability is observed as evidenced by redistribution of the fluorescence to a diffuse, cytoplasmic pattern. The loss of lysosomal membrane impermeability is correlated with Aβ1‐42 accumulation, since incubation of the cells with the nonaccumulating isoform of amyloid, Aβ1‐40, does not induce leakage. The same results were obtained using the release of soluble lysosomal hydrolases, cathepsin D and β‐hexosaminidase, into the cytosol as an assay for the leakage of lysosomal contents. Together, our results suggest that the loss of lysosomal membrane impermeability may be an early event in Aβ pathogenesis, and provide an explanation for the miscompartmentalization of extracellular and cytoplasmic components observed in Alzheimer's disease (AD). The release of hydrolases may further cause the breakdown of the cytoskeleton and the blebbing of the plasma membrane, and the leakage of heparan sulfate glycosaminoglycans from the lysosome may ultimately promote the assembly of tau into neurofibrillary tangles (NFT). J. Neurosci. Res. 52:691–698, 1998. © 1998 Wiley‐Liss, Inc.