BM 13.177, a sulphonamide derivative, prevented platelet aggregation by thromboxane A2 in vitro and selectively inhibited contraction of isolated rabbit femoral arteries induced by two stable endoperoxide analogues. In a double-blind placebo-controlled study, oral BM 13.177 inhibited platelet aggregation induced by arachidonic acid, low dose collagen, and the two stable endoperoxide analogues, and slightly prolonged the bleeding time. Generation of thromboxane or of other prostaglandins was not affected. No side-effects were seen. BM 13.177 appears selectively and safely to block platelet and vessel wall thromboxane receptors and should be useful in elucidating the role of thromboxane A2 in disease.