Bromodeoxyuridine incorporates into B lymphocytes in vivo. Two PL (BKL-2, Stara) and aleukemic (BKL, Wysoka) BLV-infected cattle and three controls (PBK, BK, 109322) (PBK and 109322 are represented) were injected intravenously with 3 g of BrdU, and an aliquot of blood (1 ml) was collected 6 days later. After lysis of the red blood cells, B cells were labeled with biotinylated 1H4 monoclonal antibody and streptavidin-phycoerythrin (PE) conjugate. Then, the cells were stained with anti-BrdU fluorescein isothiocyanate antibody in the presence of DNase and analyzed by two-color flow cytometry (x axis = BrdU; y axis = B lymphocytes). Ten thousand cells (lymphocytes, monocytes, and granulocytes) were acquired and PBMCs were selected by the forward/side scatter gating method. The total numbers of B cells are indicated in the upper quadrants.