Labeling of dividing cells with the thymidine analogue 5-bromo-2′-deoxyuridine (BrdUrd) has been frequently used to determine the life-span of cells in various lymphocyte subsets. In this study we address the question of whether administration of BrdUrd in vivo allows labeling of proliferating lymphocytes at early as well as later stages of maturation with high efficiency. For this purpose, the proportion of BrdUrd-labeled cells among all cells synthesizing DNA was determined in the bone marrow and spleen of mice that had received a single injection of BrdUrd or, alternatively, had been fed with BrdUrd for 1 week. On the average, 87% of bone marrow and 83% of spleen cells in the S phase of the cell cycle, as determined by DNA content, were labeled with BrdUrd 1.5 h following injection of the drug. We conclude from these data that administration of BrdUrd in vivo allows labeling of the vast majority of the dividing cells in both primary and secondary lymphoid organs and thus represents an efficient method to determine the life-span of lymphoid cells in vivo.