Abstract Accretion of cholesterol ester was studied in rat aortic smooth muscle cells in culture. Confluent multilayers of smooth muscld cells were exposed to human low density lipoprotein (LDL) and chloroquine and this treatment resulted in a very marked increase in cellular cholesterol ester. The degree of enrichment in cholesterol ester was related inversely to the cell density in the petri dish and was maximal in 48 h. The morphological changes after 48 h incubation with chloroquine and LDL consisted of accumulation of numerous membrane-bound inclusions containing electron-dense and electron-lucent material, some of which resembled secondary lysosomes. These changes resembled some of the changes observed in human and experimental atheromatosis. Similar inclusions were seen also in cultured human skin fibroblasts which accumulated large amounts of cholesterol ester during 48 h incubation with LDL and chloroquine. Removal of the accumulated cellular cholesterol ester was studied in the two cell types and it was markedly enhanced in the presence of lipoprotein-deficient serum and high density apolipoprotein-Sphngomyelin mixture. The morphological findings after 24 h of post incubation revealed the presence of empty vacuoles, membrane whorls and cytoplasmic lipid droplets. The present results indicate that aortic smooth muscle cells in culture can serve as a good model to study the role of the lysosomal system in atherogenesis.