['To investigate the direct effects of the Flt3 ligand (FL) on hematopoiesis, such as the stimulation of the formation of hematopoietic colonies and the proliferation of dendritic cells, as well as the indirect stimulation of hematopoiesis, especially via the proliferation of endothelial cells.', 'Mononuclear cells from human cord blood were plated in methylcellulose medium containing different cytokines to induce hematopoietic colony formation. Dendritic cells (DCs) were induced from the mononuclear cells with a cytokine cocktail with or without recombinant human soluble FL (rhFL; 100 ng/ml). The Flt3 receptors on the surface of a human microvascular endothelial cell line (ECV) were analyzed by flow cytometry. The proliferation of ECV stimulated by rhFL was measured with the microculture tetrazolium assay. The levels of FL, IL-6, IL-8, G-CSF and GM-CSF in the supernatant of ECV cultures were measured by enzyme linked immunoabsorbent assay (ELISA).', 'rhFL stimulates colony formation from cord blood when used as a sole stimulant. FL in combination with other cytokines increased colony formation significantly. The number of DCs was approximately 2.5 times higher when rhFL was used. rhFL stimulates the proliferation of ECV on which Flt3 receptors are expressed. Furthermore, ECV secretes FL, IL-6, IL-8, G-CSF and GM-CSF, which were augmented by tumor necrosis factor-alpha and rhFL.'] Does recombinant human Flt3 ligand exert both direct and indirect effects on hematopoiesis?