Inflammation accelerates polymorphonuclear leukocytes (PMN) release from the bone marrow and these PMN are implicated in inappropriate tissue injury. We have previously developed a method using 5'-bromo-2'-deoxyuridine (BrdU) to study PMN kinetics using a immunocytochemical grading system of PMN on cytospin slides. The aim of this study was to develop a flow cytometric method to quantify the number of positively stained PMN, and grade the intensity of staining for the transit time calculation of PMN through the marrow. Dividing myeloid progenitors in the marrow of rabbits were labeled with a pulse dosage of intravenous BrdU. BrdU-labeled PMN (PMN^BrdU) were detected in the circulation using a FITC-conjugated anti-BrdU monoclonal antibody. The PMN^BrdU were graded into 5 groups according to their FITC-intensity, and the transit times of PMN at different stages of development in the marrow were calculated. Results were compared using parallel immunocytochemical analysis of the same samples. In control animals, PMN^BrdU in the circulation peaked at 72 h after BrdU labeling with 36.0% of PMN labeled. In normal rabbits, the transit times of PMN through the mitotic pool (49.5 ± 4.2 h) and maturation pool (65.5 ± 3.1 h) correlated well with immunocytochemical analysis and previously published values. Using this method, we demonstrated that exposure to air pollution particles accelerates the release of PMN^BrdU from the marrow. We conclude that a flow cytometric approach for identifying BrdU-labeled leukocytes provides an objective and accurate method for studying leukocyte kinetics and behavior.