The circulating neutrophils must slow down, adhere to the vessel walls, and migrate out of the microvasculature into the tissue and air spaces to defend the lung against microorganisms. The present study was designed to provide quantitative information about each of these steps. Streptococcus pneumoniae was instilled into the left lower lobe of New Zealand White rabbits to induce a pneumonia, and this lobe was compared with the same region of the opposite lung. The distribution of blood flow was determined by using radiolabeled macroaggregated albumin, and the patterns of perfusion within the capillary bed were quantitated using Monastral blue. The number of neutrophils delivered to the pneumonic site was determined by multiplying the circulating neutrophil count and blood flow. The results show that retention of 51Cr-labeled neutrophils was increased in the pneumonic region 2, 4, and 8 h after instillation of the organisms. The number of intracapillary neutrophils was increased in the pneumonic regions at all time points and in the control regions at 1 and 4 h. Neutrophil migration occurred in the pneumonic site, but only 1–2% of the total neutrophils delivered to the region migrated out of the pulmonary vessels into the air space. We conclude that the circulating neutrophils undergo a generalized response that increases their margination throughout the lung, that increased margination in the pneumonic site changes the distribution of capillary flow, and that the majority of neutrophils delivered to a pneumonic site are returned to the circulation without migrating into the air space.