The different mechanisms underlying the antithrombotic effects of heparin and dextran are known. To document the clinically suspected potentiation effect when the two agents are used together, the following study was undertaken:In 9 patients with peripheral vascular disease, the Ivy bleeding time, the Lee-White clotting time and the platelet adhesiveness by Hellem method, were determined at control periods and at 1 and 3 hours following infusion of various doses of heparin alone. These were repeated on separate days after infusion of 500 ml of dextran in combination with same doses of heparin.While dextran alone had no effect on the clotting time, it produced a marked and statistically significant potentiation of heparin effect (P<0.01). The mean clotting time one hour after infusion of 10,000 u of heparin was increased from 36 min. with dextran to 69 min. with dextran. This effect persisted at 3 hours. The mean clotting time after 5,000 u of heparin with dextran was almost the same as after 10,000 u of heparin with dextran. The potentiation was slight at 2.500 u of heparin. Heparin did not affect platelet adhesiveness. Dextran suppressed it when it was abnormally high, but did not reduce it below the normal value of 41 ± 9%. There was no demonstrable changes in the bleeding time by either dextran or heparin at the dosage given.Our clinical experience and reports of other investigators suggest the desirability of the use of the two drugs in certain circumstances. To be effective and yet safe, it is recommended that whenever heparin is used in patients receiving dextran the dosage should be reduced to a half or a third of the usual.