The circulating white blood cells of patients with brain infarction were labelled in vitro with Indium-111 tropolonate; the cells were reinjected to study the inflammatory process by gamma camera imaging. Eight patients with acute cerebral ischemic infarct were studied during the first two weeks after the onset of neurological symptoms. In seven cases a well defined area of increased radioactivity was revealed in the infarcted hemisphere indicating active migration and tracking of labelled leukocytes in cerebral infarct. This method allows monitoring of the cellular inflammatory response in human cerebral infarcts and adds another imaging technique.