We have identified a population of cells in murine bone marrow that has many of the phenotypic characteristics attributed to resting hematopoietic stem cells but does not reconstitute irradiated mice. These cells express high levels of Sca-1, H-2K and CD38 and low levels of Thy-1.1, but do not express CD34 nor any of the lineage markers including CD3, CD4, CD5, CD8 NK1.1, I-A, B220, Ig(MGA), CD40, kappa, Mac-1, Gr-1 or Ter119. In addition, this population can be found at normal frequency in nu/nu as well as rag-1(- /-) mice. These cells incorporate only low levels of Rh123, are resistant to the cytotoxic effects of 5-fluorouracil and, consistent with their resting phenotype, less than 2% of these cells are in the S/G2/M phases of the cell cycle. The only phenotypic characteristic that distinguishes these cells from the lineage- Sca-1+, Thy-1.1(low) long-term reconstituting hematopoietic stem cell population is their lack of c-kit expression. Here we have explored the possibility that these cells represent a truly resting population of hematopoietic stem cells. We found that the lineage-, Sca-1+, c-kit- cells do not respond to hematopoietic growth factors in vitro, either alone or in combination with stromal layers. Furthermore, these cells do not form in vivo spleen colonies nor do they have the ability to reconstitute irradiated mice. Thus, this population may represent either a population of resting stem cells for which we lack the appropriate activating stimulus, or simply represent a 'mystery population' that phenotypically mimics most of the physical properties of resting stem cells. Given the close phenotypic similarity of the c-kit- mystery population cells to the c-kit+ long-term reconstituting stem cells, investigators must be rigorous to exclude their effects from other stem cell assays.