Abstract-The mutant form of human apoA1, known as apoA1 Milano, is formed as a result of arginine 173 to cysteine substitution and inhibits experimental atherosclerosis in cholesterol-fed animals. This study was designed to determine if apoA1 Milano would modify arterial thrombogenesis. Sprague Dawley rats were intravenously administered the carrier alone (nϭ8) or apoA1 Milano (20 mg ⅐ kg Ϫ1 ⅐ d Ϫ1 for 4 to 10 days, nϭ17). The abdominal cavity was opened, and the abdominal aorta was isolated. Whatman paper impregnated with 35% FeCl 3 was wrapped around the surface of the aorta, and aortic flow was recorded continuously. In carrier-treated rats, an occlusive platelet-fibrin-rich thrombus was formed in 21.2Ϯ4.1 (meanϮSD) minutes. Treatment of rats with apoA1 Milano markedly delayed time to thrombus formation (38.8Ϯ11.9 versus 21.2Ϯ4.1 minutes, PϽ0.01), inhibited platelet aggregation (25Ϯ7% versus 50Ϯ11%, PϽ0.01), and reduced weight of the thrombus (18.5Ϯ1.8 versus 23.7Ϯ2.3 mg/cm, PϽ0.01). Total cholesterol and HDL levels remained similar in both groups of rats, but plasma apoA1 Milano levels were elevated in apoA1 Milano-treated rats. In in vitro studies, incubation of platelets with apoA1 Milano reduced ADP-induced platelet aggregation by about 50%, but apoA1 Milano had no direct effect on vasoreactivity. This study provides further evidence for critical role of platelets in thrombosis. Use of apoA1 Milano offers a novel approach to inhibit arterial thrombosis. (Arterioscler Thromb Vasc Biol. 1999;19:378-383.)