In vitro studies done on blood taken from healthy volunteers showed that prostaglandin (PG) E1 consistently inhibited platelet-to-glass adhesiveness significantly. It also inhibited platelet aggregation induced by adenosine diphosphate by norepinephrine and by adenosine triphosphate. Although PGE1 itself did not affect platelet electrophoretic mobility it inhibited the changes due to adenosine diphosphate and norepinephrine. In vivo studies conducted in rabbits showed that PGE1 suppressed platelet thrombosis formation PGE1 is found in many organs and its role in the natural history and treatment of artherial thrombosis merits further study.