Leukocyte rolling in mesenteric venules 4 hours after infusion of CD40L^-/- platelets. The number of leukocytes rolling per minute was determined by phase-contrast intravital microscopy. Infusion of activated CD40L^-/- platelets induced increased leukocyte rolling compared with resting platelets indicating that platelet CD40L is not activating the endothelium (P < .01). Average ± SEM is shown.