The practicalities of gene transfer therapy using retroviral vector systems require both that host cells be as primitive as possible and that those cells be proliferating. Here, the kinetics of hemopoietic stem cells with marrow repopulating ability (MRA) have been studied with a view to defining the timescale over which these normally quiescent cells can be triggered into cell cycle. Mice were injected with hydroxyurea (1 g/kg) four times over a period of 26 h and assayed at intervals up to eight days for 8-day and 13-day spleen colony-forming units (CFU-S) and for generation of 12-day CFU-S in the bone marrow (MRA assay). The proliferative activity of these cell populations was measured by in vitro tritiated thymidine assays. CFU-S were reduced rapidly to 11% of normal and induced into cycle. Their number and proliferative quiescence recovered by four to five days. Cells with MRA reached their nadir after four days and only then started to proliferate. For each of these progenitor cell subclasses, the proliferative activity inversely reflects their numbers and indicates regulation by negative feedback processes.