Neutrophils have been implicated in the pathogenesis of several inflammatory lung disorders including asthma, emphysema, and adult respiratory distress syndrome. The precursors of these destructive cells are thought to be marginating neutrophils that, although intravascular, remain intimately associated with endothelium, resisting the shearing forces of flowing blood. The purposes of this study were (1) to examine a method for quantitating marginating lung neutrophils, (2) to assess the adherence of marginating neutrophils to normal lung endothelium, and (3) to determine the reversibility of neutrophil-endothelial cell adherence. Rats were anesthetized and ventilated, and their lungs were exposed through a median sternotomy. The left lung was tied off with blood in situ and the right lung was perfused intraarterially with colloid for 2 minutes. In separate experiments, both left and right lungs were perfused intraarterially with colloid for 25 minutes. Myeloperoxidase, a neutrophil granule enzyme, was related to lung neutrophils. Dithionite-sensitive optical density (DSOD) was related to lung hemoglobin. Marginating lung neutrophils were quantitated by measuring the peroxidase activity of normal blood-perfused lung (myeloperoxidase assay) and subtracting from it the fraction of activity corresponding to the lung blood content (DSOD). The marginating neutrophil pool was identified by 2.1 units myeloperoxidase (5 x 10(6) neutrophils) per gm wet lung. Although the marginating pool was depleted by 54% during 2 minutes of lung colloid perfusion, lung erythrocytes (DSOD) were decreased by a significantly greater 93% (p less than 0.05). Lungs perfused with colloid for 25 minutes had negligible remaining myeloperoxidase activity. We conclude that (1) marginating lung neutrophils can be quantitated with the myeloperoxidase and DSOD assays, (2) marginating neutrophils are relatively adherent to normal lung endothelium compared with intravascular erythrocytes, and (3) the adherence of marginating neutrophils is fully reversible.