To elucidate the role of apolipoprotein E (apo E) in atherogenesis, we have investigated the localization of apo E in normal and atherosclerotic aortas as well as in other tissues of 32 post-mortem individuals. Using double immunofluorescence it has been found that normal intima of individuals older than 20 years and some adolescents contained immunoreactive material that reacted with poly- and monoclonal antibodies to apo E. A staining pattern of apo E differed from that of apolipoprotein B, the latter being seen in normal intima of each child older than 7 years. Apo E was present extracellularly in lipid streaks and atheromatous plaques, where its staining was particularly intensive around the necrotic zone of plaques. Some macrophages in the plaques of 4 aortas exhibited apo E-positive staining, while aortic endothelial and smooth muscle cells never contained apo E. Apo E-positive staining was not found in the majority of vessel cells, it was always, however, observed in other types of cells including hepatocytes. Kupffer cells, spleen macrophages and cerebral astrocytes. Our findings indicate that only some macrophages in human aorta may be responsible for the production of apo E that can participate in reverse cholesterol transport. At the same time, apo E accumulation in the aortic wall may promote the development of atherosclerosis.