Defibrotide is a polynucleotide extracted from mammalian lung, which shows antithrombotic and anti-ischaemic activity in animals, probably related to stimulation of fibrinolysis and/or enhancement of prostacyclin production. The effect of a single infusion of defibrotide on fibrinolysis and the levels of certain prostanoids in man has been investigated in a cross-over double-blind placebo-controlled study. Evaluation of changes in fibrinolysis was difficult because of the spontaneous activation observed after placebo. However, the fast-acting plasminogen activator inhibitor was decreased only at end of the defibrotide infusion, suggesting a moderate profibrinolytic effect superimposed on the spontaneous activation. There was a marked and prolonged elevation of the plasma level of 6-keto-PGF1 alpha, the stable metabolite of prostacyclin. In collagen stimulated whole blood, both 6-keto-PGF1 alpha and prostaglandin E2 production were also greatly increased, with no consistent indication of inhibition of thromboxane B2. It is suggested that defibrotide stimulates prostacyclin and prostaglandin E2 production by leucocytes or via platelet/leukocyte interactions. The effects observed here should be useful in guiding subsequent clinical trials.