BACKGROUND: Umbilical cord blood (UCB) cells are an alternative source of hemopoietic stem cells for transplantation. However, the use of UCB is limited by the lack of number of hemopoietic stem cells to reconstitute bone marrow of adults or higher weight recipients. The aim of this study was to evaluate the ex vivo expansion effect of various cytokines, especially thrombopoietin (TPO) in isolated CD34 positive UCB cells. METHODS: CD34 positive UCB cells were isolated by biotin-avidin immunoaffinity method. Isolated CD34 positive cells were cultured with various combination of cytokine including TPO, SCF, IL-3, & GM-CSF. CFU-GM, CFU-Mix, and CFU-Meg colony assays were performed. RESULTS: After 3 weeks of culture, the maximal expansion of total nucleated cells was seen in the combination of TPO+SCF+IL-3. After 1 week of culture, the number of CFU-GM was significantly increased in the combination of TPO+IL-3 (P<0.05) or SCF (P<0.05). In the presence of TPO+IL-3, significantly increased number of CFU-Mix was observed (P<0.05). TPO alone enhanced colony formation of CFU-Meg. In the combination of TPO+SCF, the number of CFU-Meg was increased significantly (P<0.05). CONCLUSIONS: TPO expanded the number of CFU-Meg as well as CFU-GM and CFU-Mix. These results suggest TPO acts not only in the megakaryocytopoiesis but also in the early stage of erythropoiesis and granulopoiesis.