Apolipoprotein A-I and apolipoprotein A-II, the two major protein components of the high-density lipoproteins, were visualized in human arteries using an immunofluorescence technique. Apolipoprotein A-I and apolipoprotein A-II were codeposited into the intima and upper media of normal arteries of atherosclerotic patients. The amount of deposits increased in fatty streaks. In atherosclerotic plaques, apolipoproteins accumulated around the necrotic material. These two apoproteins were present in the extracellular matrix as well as in the foam cells surrounding the atherosclerotic lesions. The concomitant intracellular localization of apolipoprotein A-I and of apolipoprotein A-II in the cytoplasm of foam cells supports the hypothesis that extracellular high-density lipoprotein particles are internalized in the macrophages during the atheromatous process.