Bone marrow is the primary source of hematopoietic progenitors. These progenitors can be readily isolated from the mononuclear cell (MNC) fraction. Bone marrow mononuclear cells are available from a large and varied donor pool. Mononuclear cells are prepared by centrifugation in a density cell separation medium from bone marrow. Isolating the MNC eliminates the erythrocytes and graulocytes, producing a more stable cell product. MNC can be used directly in hematopoietic assays, as the starting material for isolating CD34+ progenitor cells.