PURPOSE: Umbilical cord blood is increasingly being used in the setting of allogeneic marrow transplantation. However, while neutrophil engraftment is comparable to that of marrow transplants, delayed platelet engraftment is often a concern for cord blood transplant recipients. This delay may be due to relative weakness of the megakaryocyte lineage in cord blood. We evaluated the potential of ex vivo expansion and clonality from different stem cell sources. METHODS: The CD34 cells from bone marrow (BM), umbilical cord blood (CB), and mobilized peripheral blood (PB) were cultured for burst-forming unit of erythrocyte (BFU-E), colony-forming unit of granulocyte and monocyte (CFU- GM) and colony-forming unit of megakaryocyte (CFU-MK) at day 0, day 4, day 7, and day 14 under the combination of growth factors, with cell counts. Cytokines included recombinant human megakaryocyte growth and development factors (100 ng/mL), interleukin-3 (10 ng/mL), stem cell factor (100 ng/mL), and flt-3 ligand (50 ng/mL). RESULTS: CB-derived CD34 cells had significantly higher total cell proliferation than either BM or PB at day 7 (1.6 to 18.2 fold) and day 14 (1.2 to 17.2 fold). The colony count of BFU-E was in general more plentiful in CB than in BM and PB at day 4, day 7 and day 14, among which the difference was the most distinct at day 7 culture. Also, CB CD34 cells produced more CFU-Mk colonies than did BM or PB at day 4 and day 7. There were no differences in colonies count of BFU-E and CFU-Mk between BM and PB. CONCLUSION: Ex vivo expansion of CB cells may be most promising in producing total cellular expansion, CFU-Mk and BFU-E compared with BM and PB, especially at day 7, because the former was the most productive hematopoietic source on a per volume basis.