Thrombopoietin (TPO) is the primary regulator of megakaryocyte and platelet production in vitro and in vivo. It supports also survival and proliferation of hematopoietic stem cells. The TPO receptor, c-mpl, is a member of the protooncogene family. Our studies focused on the effect of TPO on proliferation and differentiation of rhesus monkey embryonic stem cells to hematopoietic cells. The rationale of the present investigations was the finding that patients with congenital amegakaryocytic thrombocytopenia (CAMT) reveal c-mpl mutations leading to the development of pancytopenia, suggesting that c-mpl is expressed in early hematopoiesis. Here we demonstrate that rhesus monkey embryonic stem (ES) cells are capable of differentiating into uncommitted progenitor cells, including hemangioblasts (hematopoietic and endothelial precursor cells). The combination of TPO and vascular endothelial growth factor (VEGF) significantly increases the number of hemangioblasts and promotes even differentiation to CD34(+) hematopoietic progenitor cells (up to 8%). In addition, analysis of gene expression during hemangioblast development demonstrates that TPO is capable of increasing the mRNA expression of the VEGF receptor (VEGFR) and its own receptor (c-mpl). The in vitro differentiation of rhesus monkey ES cells provides an opportunity to better understand the mechanism of TPO function in the early stage of hematopoietic development from ES cells to mature hematopoietic cells.