Background and Purpose-A growing body of evidence suggests that inflammatory processes are involved in the pathophysiology of stroke. Phagocyte cells, involving resident microglia and infiltrating macrophages, secrete both protective and toxic molecules and thus represent a potential therapeutic target. The aim of the present study was to monitor phagocytic activity after focal cerebral ischemia in mice. Methods-Ultrasmall superparamagnetic particles of iron oxide (USPIO) were intravenously injected after permanent middle cerebral artery occlusion and monitored by high resolution MRI for 72 hours.