In order to determine the effect of depleting circulating polymorphonuclear neutrophils (PMN's) on brain microcirculation and lesion size in an acute stroke model, Spontaneously Hypertensive Rats (SHR) were injected intraperitoneally with either 2 ml RP-3 antineutrophil antibody followed in 4 hours by MCAO (n = 5), 2 ml saline followed in 4 hours by middle cerebral artery occlusion (MCAO) (n = 6), or 2 ml saline followed in 4 hours by sham operation (n = 3). After 4 hours of ischemia or a 4 hour interval (sham-operated animals), microvascular perfusion was assessed by means of an intravascular fluorescent tracer technique: FITC-dextran and Evans blue were injected intravenously 10 seconds and 5 seconds, respectively, before decapitation. Lesion volume was calculated by interpolation from histologic sections cut from 8 predefined stereotactic levels. MCAO with the normal complement of neutrophils led to significant impairment of perfusion in nutrient vessels and a maximal ischemic lesion volume. Depletion of circulating leukocytes by RP-3 significantly attenuated the microvessel perfusion impairment and reduced the volume of ischemic brain injury.