The platelet anti-aggregating, cardiovascular and gastro-intestinal actions of a hydantoin prostaglandin analogue, BW245C have been compared with prostacyclin and PGD2 in several species. In human plasma in vitro, BW245C was 0.2 times as active as prostacyclin and 8 times as active as PGD2 in inhibiting platelet aggregation. In rat and rabbit plasma, BW245C was only weakly active but was more potent in sheep and horse plasma. Since the activity of PGD2 varied in a parallel fashion, BW245C may interact with PGD2 binding sites on platelets. The potency of BW245C as a vasodepressor also varied between species, being 0.5, 0.1, 0.06 and less than 0.02 times as active as prostacyclin in the anaesthetised dog, monkey, rat and rabbit respectively. The relative activity of BW245C as an inhibitor of platelet aggregation ex vivo was more comparable, being 0.08, 0.04 and 0.05 times as active as prostacyclin following intravenous infusion in the rabbit dog and monkey respectively. In the rabbit, BW245C was a highly selective platelet inhibitor with minimal cardiovascular actions, whereas in the dog and monkey, BW245C lowered BP in anti-aggregating doses. The potent platelet anti-aggregating actions of BW245C following parenteral or oral administration makes this hydantoin a potentially-useful anti-thrombotic prostaglandin analogue.