Recent immunohistochemical observations on atherosclerotic lesions in human aortas and coronary arteries have revealed that these lesions contain mast cells. Mast cells, better known for their actions in allergic reactions of skin and mucosal surfaces, are filled with cytoplasmic secretory granules that contain histamine, heparin and neutral proteases, notably chymase. When activated these cells degranulate, and in this way may influence lipoprotein metabolism in their immediate environment. Indeed, animal studies have suggested that exocytosed mast cell granules may carry LDL into peritoneal macrophages, and so induce their conversion into foam cells. With immunoelectron microscopic techniques, evidence was obtained suggesting that such a 'granule carrier pathway' may operate in vivo in the human arterial intima.