It was found in a long-term bromodeoxyuridine (BrdU) infusion study that two or more different subpopulations of bone marrow stem cells exist in mice. One of these subpopulations appears to be noncycling and forms approximately 10% of eight-day CFU-S. Another one, a subpopulation of slowly cycling bone marrow cells, is represented as 14- day CFU-S. The 14-day CFU-S have a regular increment in the percentage of the subpopulation entering the cycle over time, with a cell generation half-time of 21 days. The cycling status in these experiments was ascertained by in vivo continuous long-term BrdU infusion. An improved method is presented for long-term BrdU infusion with UV killing of cycled cells.