Polymorphonuclear leukocytes (PMN) labeled in vivo with 5'-bromo-2'-deoxyuridine (BrdU) in donor animals were transferred to recipients to determine the rate of clearance of labeled PMN from the circulation, their margination within the vascular space, and migration into Streptococcus pneumoniae-induced inflammatory sites. The donor animals received intravenous infusions at 25 mg/kg/day of BrdU for 7 days when cytospins of leukocyte-rich plasma (LRP) showed that 80 +/- 2.3% PMN were labeled. The BrdU labeled cells were then transfused to serum-compatible recipients as either whole blood, leukocyte-rich plasma, or PMN purified from an equal volume of whole blood. Twenty-four hours after transfer, the distribution of BrdU-labeled PMN in the lung, liver, spleen, bone marrow, and gut was determined morphometrically and by Southern blot analysis of DNA extracted from these organs. BrdU-labeled PMN transfused as either LRP or purified PMN provided no advantage over transfusing whole blood. The half-life of BrdU-labeled PMN in the recipient circulation after transfusing whole blood was 270.4 min (95% confidence intervals, 248.5 to 296.4 min). The majority of the BrdU-labeled DNA was found in the spleen, where DNA analysis showed that the white blood cells underwent programmed cell death by apoptosis. Four hours after infection with S. pneumoniae and 1 h after transfusion of labeled whole blood, BrdU-labeled PMN had migrated into the infected sites. We conclude that transfer of BrdU PMN in whole blood provides a simple, effective method of tracing labeled PMN in vivo.