Ticlopidine (Ticlide), an anti-platelet drug with a broad scope of clinical applications, is claimed to be an antagonist of adenosine diphosphate on platelet receptors. In vitro this antagonism cannot be demonstrated. Ex vivo it is detectable many hours after oral administration of the drug, perhaps subsequently to its biotransformation to an unknown metabolite. Here, we report for the first time that in patients with peripheral arterial disease and in cats with extracorporal circulation ticlopidine evokes instantaneous thrombolytic or fibrinolytic effects which are not associated with inhibition of platelet aggregation. Shortening of euglobulin clot lysis time and increase in plasma levels of tissue plasminogen activator were observed 1-2 h after oral ingestion of ticlopidine at a single dose of 500 mg. In cats ticlopidine produced instantaneous anti-thrombotic and thrombolytic effects at doses of 0.3-1 mg/kg and 10-15 mg/kg i.v., respectively. Thrombolysis by ticlopidine (10 mg/kg i.v.) was comparable to that by prostacyclin at a dose of 0.3 microgram/kg i.v. Ticlopidine at a concentration of 100 microM increased endothelial thromboresistance in vitro. The drug did not inhibit the activity of cyclooxygenase-1 or 12-lipoxygenase while it inhibited lipid autooxidation (IC50 = 18 microM) in rat liver microsomes. Our data point to a possibility that the therapeutic efficacy of ticlopidine might be associated not only with its delayed anti-platelet effects but also with its immediate thrombolytic action which is likely to be mediated by endothelial prostacyclin and tissue plasminogen activator rather than by platelet mechanisms.