Leukocyte rolling in mesenteric venules 4 hours after platelet infusion depends on platelet and endothelial P-selectin. The number of leukocytes rolling per minute was observed by phase-contrast intravital microscopy. In WT recipients, leukocyte rolling after infusion of P-selectin^-/-–activated platelets was significantly less compared to infusion of WT-activated platelets (P < .001). Infusion of WT-activated platelets in either P-selectin^-/- or PSGL-1^-/- (▪) recipients induced significantly less leukocyte rolling compared to infusion in WT recipients (P < .001). Average ± SEM is shown.