Leukocyte rolling in mesenteric venules 4 hours after platelet infusion. (A) The number of leukocytes rolling per minute was determined by phase-contrast intravital microscopy. Infusion of resting platelets, platelet releasate, or buffer did not induce increased leukocyte rolling as compared with uninjected controls. Infusion of activated platelets induced significantly increased leukocyte rolling compared to infusion of resting platelets (^*P < .001). Average ± SEM is shown. (B) Leukocyte rolling observed by fluorescence intravital microscopy on infusion of activated (left) or resting platelets (right). Rhodamine 6G was infused to label the leukocytes. Arrows point to the vessel wall. Bar = 50 μm.