This study was aimed to investigate the effect of platelet-derived microparticles (PMP) on stimulating the proliferation of granulocyte-macrophage progenitors (CFU-GM) from umbilical cord blood. Different concentrations of thrombin were adopted to activate the platelets for releasing PMP. Flow cytometry was adopted to evaluate the efficiencies of different concentrations of thrombin to produce PMP. Umbilical cord blood mononuclear cells (MNC) were obtained from healthy donors and the MNC were isolated by Ficoll density gradient centrifugation. MNC were cultured in 2.7% methylcellulose containing different concentration of PMP and colonies were counted under an inverted microscope after 7 days. The result showed that the release rate of PMP activated by 2.0, 1.5, 1.0 and 0.5 U/ml thrombin were 28.7%, 47.7%, 50.1% and 43.9% respectively. The PMP enhanced colony formation in dose-dependent manner. The number of colonies per 2 x 10(5) MNCs in groups of PMP at different concentrations (10, 50 and 100 microg/ml) were 119.8 +/- 32.2,142.8 +/- 45.2 and 180.8 +/- 85.1 respectively. The number of colonies in the groups of PMP at 100 microg/ml and 50 microg/ml were statistically significant when compared with control group (103.0 +/- 24.8) (P < 0.05). The number of colonies per 2 x 10(5) MNC in the group of PMP (10 microg/ml) was 119.8 +/- 32.2 which was higher than that in control group, but there was no statistical significance between two groups. It is concluded that platelet activated with 1.0 U/ml thrombin can get the best release efficiency of PMP and PMP can enhance the proliferation of granulocyte-macrophage progenitor cells of umbilical cord blood.