Prostaglandin D 2 was found to be a potent inhibitor of platelet aggregation. Aggregation of human platelets by ADP, collagen and prostaglandin G 2 was inhibited more strongly by PGD 2 than by PGE 1. Although ADP-induced aggregation of rabbit platelets was inhibited more strongly by PGE 1 than by PGD 2 the latter prostaglandin gave a more long-lasting inhibitory effect on platelet aggregation following intravenous or oral administration. These results coupled with the finding that PGD 2 has less hypotensive effects on the cardiovascular system than PGE 1 suggest the possible use of PGD 2 as an antithrombotic agent.