Use of [111In]granulocytes in the study of pulmonary inflammation requires study of their in vivo behavior. To study the pulmonary deposition of these cells and their ability to migrate from the capillary to the alveolus, we injected [111In]granulocytes into dogs 24 h after the induction of a right lower lobe pneumococcal pneumonia. Using external imaging, we found rapid clearance of [111In]granulocytes from the uninvolved lung (with a residual radioactivity of 24.5 +/- 4.2% at 4 h). In contrast, 83 +/- 12.4% of the initial radioactivity was present in inflamed lung at 4 h. Bronchoalveolar lavage fluid from the inflamed lung was more cellular than that from control lung, contained a greater fraction of polymorphonuclear leukocytes (82 +/- 4.1% versus 20 +/- 6.2%), and much greater cell-associated radioactivity (ratio of 423:1, inflamed to control). Autoradiography disclosed that this radioactivity was localized to consolidated alveoli and was not prominently distributed in arterioles or venules or in airways larger than 0.6 mm. We conclude that [111In]granulocytes are biologically active in the setting of acute lung inflammation.