Heat shock protein 70 (hsp70) has been detected in atherosclerotic lesions, in which endothelial cells and smooth muscle cells are involved. In a previous report we showed that Ox-LDL, a causal factor in atherosclerosis, could induce hsp70 expression in cultured human endothelial cells [Zhu et al. B.B.R.C. 1994, 200:389]. Here, with immunofluorescence and immunoblotting techniques, we show that Ox-LDL are capable of inducing hsp70 expression also in human smooth muscle cells, and that this induction is dependent on cell density and on the concentration of Ox-LDL. The induced expression of hsp70 was higher in human umbilical vein smooth muscle cells than in a human smooth muscle cell line. Conversely, Ox-LDL was cytotoxic to both types of cells, more so to the human smooth muscle cell line. These observations indicate that Ox-LDL may be a stress responsible for hsp70 expression in atherosclerotic plaques and the presence of hsp70 in plaques may be a useful marker for continuous oxidative damage in the arterial wall.