Time course of induction of leukocyte rolling in mesenteric venules after infusion of activated platelets. The number of leukocytes rolling per minute was observed by phase-contrast intravital microscopy. Infusion of activated platelets induced a significant increase in leukocyte rolling compared with uninjected mice after 2 hours and 4 hours. Leukocyte rolling decreased by 7 hours after infusion. ^*P < .01, compared to uninjected mice. Average ± standard error of the mean (SEM) is shown.