The early stages of microvascular injury are often difficult to detect due to the lack of a suitable marker to assess such an injury. We utilized the well known phenomenon of polymorphonuclear leukocyte (PMN) migration to the microvascular bed as a result of acute inflammatory reactions originating from the damaged cells. A radiotracer technique was developed, employing indium-111-labeled PMN for the detection of microvascular injury induced by hyperoxia. New Zealand white rabbits exposed to either 100% oxygen or air for various intervals of time were injected with indium-111-tropolone or oxine-labeled PMNs. Influx of radioactive PMN into the lung was detected in 72 hr/oxygen-exposed animals using gamma scintigraphic technique. Analysis of dry/wet ratios and histological examinations of the lung biopsies indicated noncardiogenic edema formation at this stage. Mortality was 50% beyond 96 hr/oxygen exposure. Our study thus provided a means to detect early microvascular injury during 72 hr/oxygen-exposure, which was not detectable by any other noninvasive techniques. The use of indium-111-labeled PMN thus appears to be a potentially important tool for the clinical assessment of lung microvascular injury.