We studied the effects of stem cell factor (SCF) and flt3 ligand (FL) on the ex vivo expansion of human umbilical cord blood (CB)-derived CD34+ cells in combination with various cytokines, including interleukin (IL)-3, IL-6, IL-11, and c-Mpl ligand (thrombopoietin, TPO), in a short-term serum-free liquid suspension culture system. Among the two-factor combinations tested, SCF plus IL-3 most effectively expanded committed progenitor cells, including mixed colony-forming units (CFU-Mix). The expansion efficiency (EE) of FL for each progenitor was inferior to that of SCF in the presence of various cytokines, except TPO. IL-6 significantly increased the EE for granulocyte/macrophage colony-forming units (CFU-GM) obtained with SCF+ IL-3 or FL+ IL-3. Interestingly, TPO markedly augmented the EE for committed progenitors, including CFU-GM, erythroid burst-forming units (BFU-E), and CFU-Mix, in the presence of SCF+ IL-3 or FL+ IL-3. The combinations of SCF+ IL-3+ TPO+ IL-6 or IL-11 maximally stimulated the expansion of committed progenitors. The maximum EE for CFU-GM, BFU-E, and CFU-Mix was respectively 197-fold (day 14), 60-fold (day 7) and 51-fold (day 14). Other combinations of cytokines without IL-3 failed to expand effectively these committed progenitors. Our data demonstrate that it is possible to expand human CB-derived committed progenitors in vitro using SCF or FL with several other cytokines including TPO, and that IL-3 is the key cytokine promoting the expansion of human hematopoietic progenitors in the presence of SCF or FL.