The long term marrow-repopulating ability of mouse bone marrow c-Kit+ Sca-1+ Lin(low/-) cells was studied. Injection of as few as 100 c-Kit+ Sca-1+ Lin(low/-) cells could rescue a lethally irradiated recipient mouse and reconstitute both myeloid and lymphoid cells in their peripheral blood for at least 10 mo. When limiting dilution analysis was performed in the presence of host-derived rescue cells, as low as five c-Kit+ Sca-1+ Lin(low/-) cells were shown to be capable of repopulating lymphohemopoietic cells. Subsequently, we examined whether c-Kit+ Sca-1+ Lin(low/-) cells had a capacity for self-renewal in vivo. After transplantation of 500 c-Kit+ Sca-1+ Lin(low/-) Ly-5.1+ cells into lethally irradiated Ly-5 congenic mice, the expansion of cells with the same phenotypes as the injected cells was monitored. By day 28 post-transplantation, more than 50,000 donor type c-Kit+ Sca-1+ Lin(low/-) Ly-5.1+ cells were found in the spleen and over 18,000 cells were found in bone marrow. The expansion in spleen, however, was transient in that by day 60 cells of the donor phenotype were found only in bone marrow. The c-Kit+ Sca-1+ Lin(low/-) LY-5.1+ cells expanded in spleen or bone marrow contained as many high proliferative potential colony-forming cells as the originally injected cells. They also contained cells with LTRA, but the frequency appeared to be less compared with naive c-Kit+ Sca-1+ Lin(low/-) cells. These data provide evidence that c-Kit+ Sca-1+ Lin(low/-) cells in bone marrow are capable of multilineage differentiation as well as self-renewal and that spleen is a primary site of stem cell expansion after transplantation.