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Aortic smooth muscle cell death is an important initial lesion of atherosclerosis. A number of autooxidation products of cholesterol which has been recognized recently has the capability of inducing rabbits' aortic smooth cell death in vitro. Twelve oxidation derivatives of cholesterol, available commercially, were dissolved in small amounts of ethanol, then added to the culture medium at levels not exceeding 0.8%. The medium contained 10% fetal calf's serum which served as an in situ vehicle for the sterols. The degrees of cytotoxicity were graded and measured as percentage of dying and dead cells in the cultures within 24 hr. 25-Hydroxycholesterol and cholesthan-3 beta, 5 alpha, 6 beta-triol, were the most toxic compounds among the sterols tested. When these oxidation derivatives of cholesterol were added to these cultured cells, they significantly depressed activity of 3-hydroxy-3-methylglutaryl coenzyme A reductase, a regulatory enzyme of cholesterol biosynthesis (up to 83% inhibition by 25 hydroxycholesterol at a 3 microgram/ml concentration in culture medium) but the sequence of degree of inhibition was not exactly correlated with that of cytotoxicity. Various mechanisms are speculated. Purified cholesterol showed no cytotoxic effect and minimal inhibition of cholesterol biosynthesis.