The extent of incorporation of bromodeoxyuridine (BrdUrd) into DNA of different types of hematopoietic stem cells assayed in vivo and of progenitor cells assayed in vitro was determined after continuous infusion of BrdUrd into mice for either four or seven days. Cells surviving subsequent exposure to 320 nm ultraviolet light (UV320) were considered not to have incorporated BrdUrd. Assays of stem cells were carried out in 8-Gy-irradiated Balb/c-specific pathogen-free (SPF) mice by measuring the ability of injected marrow: (a) to form spleen colonies at either seven, ten, or 13 days (the units giving rise to such colonies were named CFU-S-7, CFU-S-10, and CFU-S-13), (b) to increase the marrow content at 13 days of colony-forming cells (CFC) responsive in vitro to pregnant-mouse-uterus extract (P) (this ability was named marrow-repopulating ability P-CFC [MRA-P-CFC]) and, (c) to increase blood platelet counts at day 13 (this was named platelet-repopulating ability [PRA]). The in vitro assays carried out on marrow from BrdUrd-infused mice were measurements of the content of CFC responsive to P and to P plus human spleen-conditioned medium (H). The percentage survival after exposure to UV320 in marrows obtained after four and seven days of infusion of BrdUrd, respectively, was: MRA-P-CFC, 100% and 100%; PRA, 80% and 50%; CFU-S-13, 65% and 25%; CFU-S-10, 11% and 3%; CFU-S-7, 8% and 2%; P + H CFC, 20% and 12%; and P-CFC, 6% and 6%. These results are in agreement with predictions from previous experiments that studied the effects of 5-fluorouracil (5-FU) on bone marrow using the same assays and are compatible with a model that considers marrow to be organized as a concatenated series of compartments in which turnover rate increases as maturity increases.