In acute bacterial pneumonia, polymorphonuclear leukocytes (PMN) sequester in the lung and migrate into the alveolar airspaces. These local events are accompanied by a systemic response that includes release of PMN from the bone marrow into the circulation. The present study was designed to compare the sequestration and migration of these newly released PMN with those already in the circulation in a model of acute streptococcal pneumonia in rabbits. PMN were labeled in the mitotic pool in the marrow by administration of 5'-bromo-2'-deoxyuridine (BrdU 100 mg/kg) and the labeled cells were detected in blood and tissues by immunohistochemistry. The proportion of BrdU-labeled PMN (PMN BrdU) that sequestered and migrated in the lung tissue infected with Streptococcus pneumoniae and the uninfected lung was measured using morphometric techniques. The results show an increase in the proportion of PMN BrdU (6.0 +/- 1.0% to 17.3 +/- 3.8%, P<0.05) in the circulation 5 h following the induction of a pneumonia and the PMN expressed a higher concentration of L-selectin (9.3 +/- 0.7 to 14.9 +/- 0.8 MFI, P<0.05). The proportion of PMN BrdU in the control tissue was not different from the proportion in the systemic circulation (11.4 +/- 1.6%). The PMN BrdU increased in the pneumonic site at 5 h (19.9 +/- 3.4%, P<0.05) and 8 h (26.6 +/- 1.5%, P<0.05) after treatment. Only 2.8 +/- 0.3% and 2.8 +/- 0.6% of the PMN that migrated into the airspace at 5 and 8 h were PMN BrdU. We conclude that PMN released into the circulation as part of the systemic response to a local streptococcal pneumonia sequester normally but may be slow to migrate into the airspaces at the inflammatory site.