Normal human bone marrow mononuclear cells were fractionated by differential adherence, immunomagnetic separation, and fluorescence- activated cell sorting (FACS). The resultant fractionated cells were cultured in semisolid medium to monitor the presence of BFU-E, Mix-CFC, and nonerythroid CFC. Two populations of cells were recovered on the basis of binding by the monoclonal antibody (MoAb) RFB-1. One of these populations contained BFU-E that were stimulated only by erythropoietin (Epo), whereas the second population contained BFU-E responsive to Epo, Epo and recombinant human granulocyte-macrophage colony-stimulating factor (rHGM-CSF), or Epo and human placental-conditioned medium (HPCM). Prior enrichment of clonogenic cells by removal of adherent and Leu-M3+ve, Leu-4+ve, Leu-7+ve, B1+ve, WEMG1+ve, and Glycophorin A+ve cells, followed by FACS fractionation on the basis of RFB-1 binding, consistently resulted in recoveries of BFU-E, Mix-CFC, and nonerythroid CFC of greater than 100% (up to 800%). These procedures also resulted in enrichment of up to 200-fold and frequencies of 1:6 for BFU-E, 1:5 for CFC, and 1:130 for Mix-CFC.