1-(Theophyllin-7yl)-ethyl-2-[2-(p-chlorophenoxy)-2-methylpropionate] (etofylline clofibrate, ML 1024, Duolip) was shown to be a potent inhibitor of experimental thrombus formation in the microvasculature of the hamster cheek pouch and of platelet aggregation and adhesiveness in the cynomolgus monkey. Inhibitory effect on thrombus formation was superior to that obtained with either of the molecule components (clofibric acid and etofylline) or their 1 : 1 mixture and a synergistic effect was apparent. Etofylline clofibrate was more active than a commercial antithrombotic drug combination of acetylsalicylic acid (ASA) and dipyridamole as an inhibitor of thrombus formation in this test system (p less than 0.05) at the proposed therapeutic dose level for each drug. In the cynomolgus monkey, etofylline clofibrate was shown again to be a potent inhibitor of platelet aggregation induced by ADP and collagen and of platelet adesiveness after administration of 12 mg/kg/day for 21 days. A mixture of clofibrate and etofylline (21.25 mg/kg/day and 3.75 mg/kg/day, respectively) had relatively less effect on platelet aggregation and no consistent effect on platelet adhesiveness. The activity found in both test systems indicates a promising antithrombotic potential for etofylline clofibrate and warrants further investigation in humans.