['The purpose of this study was to clarify the role of fibroblast growth factors (FGFs) and FGF receptors (FGFRs) in hematopoietic/endothelial development.', 'Using several different FGFR-1-specific antibodies and FGFR-1 promoter-driven LacZ activity, we show that FGFR-1 is expressed and active as a tyrosine kinase in a subpopulation of endothelial cells (approximately 20% of the endothelial pool) during development in embryoid bodies. In agreement, in stem cell-derived teratomas, expression of FGFR-1 was detected in some but not all vessels. The FGFR-1 expressing endothelial cells were mitogenically active in the absence and presence of vascular endothelial growth factor (VEGF). Expression of FGFR-1 in endothelial cell precursors was not required for vascular development, and vascularization was enhanced in FGFR-1-deficient embryoid bodies compared with wild-type stem cells. In contrast, hematopoietic development was severely disturbed, with reduced expression of markers for primitive and definitive hematopoiesis.'] Is fibroblast growth factor receptor-1 expression required for hematopoietic but not endothelial cell development?