The origin of endothelial progenitor cells (EPCs) in umbilical cord blood (UCB) is unknown. In this study, we explored the origin of UCB-derived EPCs by culturing CD14+ or CD14- subpopulation separately and co-culturing these two subpopulations either with or without transwells. We found no colony formation with CD14+ or CD14- subpopulation alone, but there were EPC colonies observed in direct co-cultures of both subpopulations. Transwell culture system was used to further study the effect of cytokines on EPC colony formation. We observed the presence of EPC colonies derived from CD14- subpopulation in the presence of CD14+ subpopulation in the upper compartment whereas there was no colony generated from CD14+ subpopulation with CD14- subpopulation in the upper compartment. Therefore, CD14- subpopulation is likely to be the origin of EPCs and EPC colony derivation requires cytokines released from CD14+ subpopulation. We further characterized the founding population of UCB-derived EPCs by separating CD14- subpopulation into CD14-/CD34+ and CD14-/CD34- subpopulations. There were colonies observed only in co-cultures of CD14+ with CD14-/CD34+ subpopulation but not with CD14-/CD34- subpopulation either with or without transwells. We screened 42 cytokines involving in angiogenesis using an ELISA array in the supernatant collected from CD14+ compared to CD14- subpopulations. We found consistently the presence of angiogenin1 in the supernatant of CD14+ subpopulation but not in that of CD14- subpopulation. The addition of angiogenin1 in culture of CD14- subpopulation yielded EPC colonies. We conclude that UCB-derived EPCs are confined to CD14-/CD34+ subpopulation and angiogenin1 released from CD14+ subpopulation may be an important factor promoting the EPC colony formation. © 2011 Springer-Verlag.