Aggregation of human blood platelets induced by adenosine diphosphate or 1-epinephrine was inhibited when the platelets were suspended in plasma which had been previously exposed to an insolubilized omega-aminohexylagarose derivative of prostaglandin E1. This decrease of platelet aggregation was not accompanied by a change in the concentration of adenosine 3',5'-monophosphate (cyclic AMP) in platelets. The results demonstrate the existence of an alternative pathway independent of cyclic AMP for the inhibition of platelet aggregation by plasma.