The size and location of the marginated pool of neutrophils (PMNs) in rabbit lungs were evaluated, and the rate of exchange of the PMNs with the circulating pool was determined. 99mTc-labeled erythrocytes (99mTc-RBCs) and 125I-labeled macroaggregated albumin (125I-MAA) were used to determine RBC transit times in the pulmonary circulation. Radiolabeled PMNs were studied on their first passage through the lungs. After 10 min of circulation, the lungs were fixed, gamma counted, and prepared for morphometric and autoradiographic studies; 74 +/- 3% of the PMNs was retained in the lungs on the first passage, and 23 +/- 2% was within the pulmonary marginated pool 10 min later. The regional PMN retention and the rate of exchange between the marginated and circulating PMN pools in the lung were directly related to RBC transit time. The radiolabeled PMNs distributed similarly to the unlabeled cells within the microvasculature and had a similar exchange rate between the marginated and circulating pools (1.4 +/- 0.2%/s using labeled cells and 1.5 +/- 0.5%/s using unlabeled cells). The marginated pool was located primarily within alveolar capillaries and contained two to three times as many PMNs as the total circulating pool.