We investigated the effects of recombinant human thrombopoietin (rhTPO) on the growth of megakaryocytic (MK) colony derived MK progenitors from human cord blood (CB) in vitro and the effects of gestational age on the number of MK colonies. The results demonstrated that rhTPO alone supports the growth of MK colonies and induces not only proliferation but also differentiation of MK progenitors. CB shows a high frequency of MK colonies; most of which are very large and equivalent to high proliferative potential colony-forming unit-megakaryocyte. The colonies could be macroscopically observed as white spots in the culture dish. Preterm neonates showed greater numbers of MK progenitors than term neonates and there was an inverse correlation between gestational age and concentration of MK progenitors of CB. The effects of gestational age was an important factor on the proliferative capacity of MK progenitors and on the response to rhTPO.