Platelet aggregation by various inductors was studied in citrated and heparinized plasma of the following groups of subjects: Normal, hemophilia A, combined factor V and factor VIII deficiency, v. Willeprand's disease and congenital afibrinognemia. The results may be summarized as follows: A-platelet aggregation in citrated plasm 1) platelet aggregation by common inductors ADP, adrenalin and collagen was normal in all groups of subjects but for the patients with congential afibrinogenemia in whom adrenalin induced aggregation was absent or markedly refuced whereas ADP and collagen gave slightly reduced or near normal aggregation curves. 2) platelet aggregation by ristocetin was normal in all groups of subjects but for v. Willebrand's disease in which it was absent. B-platelet aggregation in heparized plasma. 1) platelet aggregation by common inductors resulted to be normal in all groups of subjects except in congenital afibrinogenemia. In this latter case the pattern was still mildly defective but here was an increased aggregation as compared to citrated plasma. These findings have been interpretemmon inductors. 2) platelet aggregation by ristocetin resulted to be absent in all groups of subjects investigated. The possible mechanism of action of the inhibitory effect exercised py heparin with regard to restocetin is discussed.