Summary. Leukocyte rolling on vascular endothelium is mediated by an interaction between P-selectin expressed on endothelial cells and P-selectin glycoprotein ligand-1 on leukocytes. This interaction reduces the velocity of leukocyte movements to allow subsequent firm adhesion and transmigration. However, the interaction has so far been observed only under low venous shear stress and cannot explain the accumulation of monocytes in atherosclerotic plaques found in arteries, where shear stress is much higher. We have previously shown that newly released ultra-large von Willebrand factor (ULVWF) forms extremely long string-like structures to which platelets tether. Here, we investigated whether platelets adhered to ULVWF strings are activated and form aggregates. We also determined whether activated platelets on ULVWF strings can support leukocyte tethering and rolling under high shear stresses. We found that platelets adhered to ULVWF expressed P-selectin and bound PAC-1, suggesting their rapid activation. We also found that leukocytes tethered to and rolled on these platelet-decorated ULVWF strings, but not directly on endothelial cells, under high shear stresses of 20 and 40 dyn/cm2 in a P-selectin dependent manner. These results suggest that the endothelial cell-bound ULVWF provide an ideal matrix to aggregate platelets and recruit leukocytes to endothelial cells under high shear stress. The observed phenomenon delineates a mechanism for leukocytes to be tethered to arterial endothelial cells under high shear, providing a potential link between inflammation and thrombosis.