The biophysical and biochemical basis for the 1 degree wave of platelet aggregation induced by ristocetin (von Willebrand factor dependent) and ADP, epinephrine and thrombin (fibrinogen dependent) will be studied. A hypothesis based on colloidal stability theory in which it is suggested that these plasma protein cofactors act as bridging molecules will be tested. The portions of these proteins required for their platelet cofactor function will be analyzed by biochemical alterations. The technique of platelet electrophoretic mobility will be applied to studying the electrostatic interactions which take place during 1 degree aggregation by these agents. Techniques will be developed for analyzing the platelet proteins which contribute charged groups to the platelet surface.