The distribution of actin, vimentin, desmin, and tropomyosin was studied in the media of the human aorta and femoral and coronary arteries, as well as in atheromatous plaques from the same arteries, by means of immunofluorescence, densitometric analysis of sodium dodecylsulfate-polyacrylamide gel electrophoresis, and bidimensional gel electrophoresis. The proportions of desmin-containing cells varied in the media of different arteries; 4 per cent of the cells in the aorta, 11 per cent in the coronary artery, and 37 per cent in the femoral artery contained desmin. In fibrous atheromatous plaques, independently of the artery, desmin-containing cells were almost absent, but they reappeared in complicated lesions. The content of vimentin per smooth muscle cell increased in fibrous atheromatous plaques, whereas the content of actin and tropomyosin was less than in normal media. Moreover, the alpha-actin predominance observed in the media was transformed to beta-actin predominance in the atheromatous plaques. These cytoskeletal changes provide new, possibly useful, biochemical markers for the characterization of smooth muscle cells during early and advanced phases of atheroma formation.