A life threatening blood clot is the major cause of heart attacks. Thrombolytic therapy attempts to restore blood flow by activating the body's own fibrinolytic system at the site of the occlusive thrombus. However, for unknown reasons, therapy is unsuccessful in greater than 20% of patients. We have previously shown that the endothelial cells lining the wall of the vessel can play a substantial role in inhibiting clot lysis. This is due chiefly to the secretion of inhibitory molecules by endothelial cells. However, endothelial cells also have receptors for fibrin and little is known about how the direct interaction of fibrin with cells may influence lysis. To investigate this we have undertaken a series of microscopic studies to analyse the influence of endothelial cells on clot structure. We report here that endothelial cells can organize clot fibers into tight assemblies. We also show that, at least in culture, fibrin can act in concert with antithrombotic molecules to dramatically affect endothelial structure