Sections of human atherosclerotic plaques were stained by the indirect immunoperoxidase technique using three rat anti-human monoclonal antibodies: YAML 501.4 (anti-'leukocyte-common' (T200) antigen; YTH 8.18 (antimacrophage cytoplasm); and YPC 1/3 . 12 (anti-smooth muscle cell). The cells of diffuse intimal thickening were almost all smooth muscle cells but there were rare subendothelial macrophages. Focal lesions, in contrast, contained numerous macrophages as well as smooth muscle cells. Macrophage 'foam cells' were particularly numerous in fatty streaks and in advanced fibro-fatty plaques, but were less conspicuous in focal fibro-elastic lesions. The results confirm that macrophages are an important component of atherosclerotic plaques and suggest that they may have a significant role in atherogenesis in man.