The cytotoxicity of oxidized low density lipoprotein (oxLDL) to artery wall cells might contribute to atherosclerosis by causing cell death, presumably by both apoptosis and necrosis. After its uptake into macrophage lysosomes, oxLDL is poorly degraded, resulting in ceroid-containing foam-cells. We studied the influence of oxLDL on lysosomal enzymatic activity and, in particular, on lysosomal membrane stability, and the modulation of these cellular characteristics by high density lipoprotein (HDL) and vitamin E (vit-E). J-774 cells were incubated for 24 to 48 h under otherwise normal culture conditions with either only oxLDL or with oxLDL together with HDL or vitamin E. The lysosomal marker enzymes cathepsin-L and N-acetyl-β-glucosaminidase (NAβGase) were biochemically assayed after cell fractionation. A test for lysosomal integrity, estimating the uptake of the lysosomotropic weak base acridine orange (AO), was applied as well. Cathepsin-D was immunocytochemically demonstrated in J-774 and human monocyte-derived macrophages (HMDMs). We found that the total activities of NAβGase and cathepsin-L were significantly decreased, whereas their cytosolic activities were enhanced after oxLDL-exposure. Labilization of the lysosomal membranes was further proven by decreased AO-uptake and relocalization to the cytosol of cathepsin-D, as estimated by light- and electron microscopic studies and immunocytochemistry. It was further found that HDL and vitamin E significantly diminished the cytotoxicity of oxLDL. The results indicate that endocytosed oxLDL not only partially inactivate lysosomal enzymes, but also destabilize the acidic vacuolar compartment causing relocalization of lysosomal enzymes to the cytosol. This in turn may lead to foam cell formation.