Chylomicron remnants were found to be cytotoxic to cultured arterial smooth muscle cells. Cell death was estimated by two methods: colourimetric assay using a tetrazolium salt ahd trypan blue staining. Both methods showed considerable cell death. Twenty-five micrograms of cholesterol/ml of chylomicron remnants appeared not to injure smooth muscle cells; however, greater concentrations (= and >50 microg cholesterol/ml) caused considerable damage. Our observations may offer a possible cause of cell death in atherosclerotic plaques.