The formation of ULVWF–platelet strings under flow condition was blocked by anti–P-selectin antibody or soluble P-selectin. HUVECs were stimulated with histamine in the absence or presence of a polycloncal anti–P-selectin antibody, purified human P-selectin, the monoclonal anti-α_vβ₃ antibody LMP609, RGDS peptide, or heparin for 15 minutes at room temperature. Washed platelets suspended in Ca⁺⁺, Mg⁺⁺-free Tyrode buffer were then perfused over the stimulated endothelial cells at 2.5 dyn/cm² for 2 minutes at 37°C. The number of ULVWF–platelets strings was then counted in 20 continuous fields of 400 × (A, bar = 100 μm). Results in (D) are from 12 separate experiments. Data are mean ± SEM.