Inhibition of platelet aggregation by contact factor eluate (CFE) is demonstrated. A similar inhibition of platelet aggregation is found following incubation of ellagic acid with native platelet rich plasma. Inhibition of platelet aggregation by CFE or ellagic acid was not found when PRP from a patient with Hageman’ disease was used. The electrophoretic mobility of platelets was increased by the action of CFE. The possible role of the contact factor as a regulator of the platelet plug in the early stages of haemostasis is discussed.