Thrombopoietin (TPO) is known to sensitize platelets to other agonists at 20 ng/ml, and above 100 ng/ml it is an independent activator of aggregation and secretion. In studies with a perfusion chamber, TPO, between 0·01 ng/ml and 1 ng/ml, increased platelet adhesion to surface‐coated fibrinogen, fibronectin and von Willebrand Factor (VWF) but not to a collagen‐coated surface. Increased adhesion was observed at shear rates of 300/s and 800/s in perfusions with whole blood as well as in suspensions of platelets and red blood cells reconstituted in plasma. The by the cyclooxygenase inhibitor, indomethacin, and the thromboxane A₂‐receptor blocker, SQ30741, abolished the stimulation by TPO. The effect of TPO was mimicked by a very low concentration (10 nmol/l) of the thromboxane TxA₂ analogue, U46619. Real‐time studies of platelet adhesion to a VWF‐coated surface at a shear of 1000/s showed that about 20% of the platelets were in a rolling phase before they became firmly attached. TPO (1 ng/ml) pretreatment reduced this number to < 5%, an effect again abolished by indomethacin. Thus, TPO potentiates the direct and firm attachment of platelets to surface‐coated ligands for α_IIbβ₃, possibly by increasing the ligand affinity of the integrin.