Hematopoietic progenitors obtained from the bone marrow of healthy adults fail to undergo clonogenic maturation in vitro if a source of hematopoietic growth factors is not included in the culture dishes. In contrast, a fraction of similarly purified progenitors obtained from umbilical cord blood undergo clonogenic maturation even in the absence of added growth factors. We postulated that production of hematopoietic growth factors within the culture dishes containing the progenitors of umbilical cord blood origin might be responsible. We postulated further, that this production might be by non-progenitor cells co- plated along with the progenitors, or alternatively by CD34+ cells themselves, or by cells clonally derived from CD34+ cells. To test these possibilities we first assessed the effect of including in the cultures neutralizing antibody directed against various growth factors. Inclusion of anti-granulocyte macrophage colony-stimulating factor (GM- CSF) and anti-interleukin-3 (IL-3) (but not anti-IL-2) significantly reduced the growth factor independence of cord blood progenitors (P > .005 and P > .01). Inclusion of both anti-GM-CSF and anti-IL-3 almost completely ablated the spontaneous colony growth (P > .001). Inclusion of IL-10 also reduced, in a concentration-dependent fashion, the spontaneous generation of umbilical cord blood-derived colonies. Transcripts for GM-CSF and IL-3 were detected, by reverse transcriptase- polymerase chain reaction (RT-PCR), in the CD34+ cells from cord blood and from adult marrow. When plated without added growth factors, however, the CD34+ cells of adult marrow origin failed to produce colonies, whereas 6% of cord blood CD34+ cells similarly cultured did so. When these growth factor independent colonies were plucked from culture, transcripts for GM-CSF and IL-3 were identified in all. We conclude that production of GM-CSF and IL-3 occurs within culture dishes containing hematopoietic progenitors of umbilical cord origin, and that this explains some of their apparently unique features of in vitro growth.