We previously reported that CD31bright cells, which were sorted from cultured AC133+ cells of adult peripheral blood cells, differentiated more efficiently into endothelial cells than CD31+ cells or CD31- cells, suggesting that CD31bright cells may be endothelial precursor cells (EPCs). In this study, we found that CD31bright cells have a strong ability to release cytokines. The mixture of vascular endothelial growth factor (VEGF), thrombopoietin (TPO), and stem cell factor (SCF) stimulated ex vivo expansion of the total cell number from cultured AC133+ cells of adult peripheral blood cells and cord blood cells, resulting increment of the adhesion cells, which endothelial NO synthase (eNOS) and KDR were positive. Moreover, the mixture of VEGF and TPO increased the CD31bright cell population compared with VEGF alone or the mixture of VEGF and SCF. These data suggest that TPO is an important growth factor that can promote EPCs expansion ex vivo.