Cell proliferation in long-term suspension cultures of mouse bone marrow was studied. Bone marrow cells were maintained on irradiated or conventional non-irradiated semiallogeneic adherent feeder layers and the origin of the proliferating cells was investigated by using the unique T6 chromosome marker. In cultures containing irradiated feeder layers total cell numbers were less at the first week after "recharging," but the levels soon reached those of conventional non-irradiated cultures. At the first weeks after recharging, proliferating cells, maintained on conventional non-irradiated feeder layers, originated mainly from the adherent layer. The contribution of the recharging population to the proliferating cell population increased with time. In contrast, the proliferating cells grown on irradiated adherent layers originated mainly from recharging population from the first week after recharging. The origin of the nonadherent CFUs and their development in F1 hybrid mice and mice of syngeneic strains was also examined. The pluripotent stem cells grown on non-irradiated feeder layers were derived mainly from the adherent layer.