Using our original assay system we found that ticlopidine (TP, 30 mg/kg i.v.) had produced a prompt thrombolysis of preformed clots in extracorporeal circulation of anaesthetized rats. In contrast with ticlopidine streptokinase (SK, 30,000 U/kg i.v.) proved thrombogenic for an initial period of 10-30 min, followed only later by the expected thrombolytic action. In ticlopidine pretreated rats the thrombogenic effect of streptokinase was eliminated, and its thrombolytic potency intensified. This is the first experimental support for a conception that a combined therapy with ticlopidine and streptokinase may restrict the "early hazard" associated with streptokinase therapy.