Endotoxemia causes neutrophil sequestration in the pulmonary vascular bed. Such sequestration may be a critical initiating event in the generation of microvascular injury, although the mechanisms that lead to this localization are not understood. Toinvestigate these phenomena, the following stuelyemployed intravenous pulses of 1" Indium-tropolonate-Iabeled neutrophlls ('" In_ neutrophlls), which circulated in the rabbit with normal kinetics and responded in a manner Indistinguishable from unlabeled, circulating neutrophlls In response to an Intravenous injection of purified endotoxic lipopolysaccharide (LPS) or epinephrine. Pulmonary sequestration of'''In-neutrophlls was assessed by quantitative external gamma camera scintigraphy of a lung suprahllar region of interest. Noninvasive assessment of radioactivity by this method accurately reflected total lung radioactivity, which was shown by autoradiography to be confined to the Injected 1" In-neutrophlls.