Myeloid and erythroid progenitor cells were enriched from human marrow by selecting CD34-positive (CD34 + ve) cells, labeled with the My10 (HPCA-1) antibody, using a fluorescence-activated cell sorter. Seventy-one percent of CD34 + ve cells were blasts and most of these were too primitive to be identified by standard morphological criteria. On average, 9.5% of CD34 + ve cells formed clones after 14 days of culture in semisolid medium supplemented with erythropoietin and medium conditioned by 5637 bladder carcinoma cells. Over 2.5% of CD34 + ve cells were day-14 myeloid colony-forming cells and 2.4% were erythroid colony (burst)-forming progenitors. The remaining progenitors formed myeloid and erythroid clusters. A subpopulation of day-14 myeloid colony-forming cells failed to respond to recombinant human granulocyte-macrophage colony-stimulating factor (rhuGM-CSF) after accessory cells were removed during enrichment, so it appears that this factor can induce myeloid growth indirectly as well as directly. Recombinant human GM-CSF also supported erythroid colony-formation in cultures of CD34 + ve cells, which suggests that this hemopoietin may act directly on erythroid progenitors.