Apolipoprotein E (apo E) mediates both lipid accumulation by and removal from cells and may be secreted by both macrophages and smooth muscle cells in vitro, but its cellular source in atherosclerotic plaques is not known. Lipoprotein lipase (LPL) also enhances cell lipid accumulation and is synthesized by macrophage foam cells in atherosclerotic plaques. To determine the cellular source of apo E in human coronary atherosclerotic lesions and its relationship to LPL synthesis, in situ hybridization and immunohistochemistry were performed on 12 atherosclerotic plaques and six nondiseased coronary artery segments from 10 cardiac transplant recipients. Apo E messenger RNA was localized to both non-foam cell and foam cell macrophages in plaques, but not to other cell types, and was not detected in nonatherosclerotic arteries. Half of the regions with non-foam cell macrophages expressed neither apo E nor LPL messenger RNA, whereas 86% of macrophage foam cell-containing regions contained both messenger RNAs. Polyclonal antisera raised against human apo E localized apo E protein to the surface of macrophages and surrounding matrix in plaques but not in control coronary segments. An LPL-specific monoclonal antibody demonstrated that, similar to apo E, LPL protein on foam cell and non-foam cell macrophages was detected in atherosclerotic lesions, but LPL was also localized to intimal muscle smooth muscle cells and was not distributed as widely in association with matrix as was apo E. The expression of both apo E and LPL in atherosclerotic lesions but not in normal intima suggest that these molecules play a role in lipid metabolism in atherosclerosis.