The process of vasculogenesis was characterized in the 6.5- to 9.5-day mouse embryo and in allantoic culture by analysis of spatial and temporal expression patterns of the endothelial or hematopoietic lineage-associated proteins, TAL1, Flk1, platelet/endothelial cell adhision molecule (PECAM), CD34, VE-cadherin, and Tie2. The study establishes that: (1) TAL1 and Flk1 are coexpressed in isolated mesodermal cells that give rise to endothelial cells and thus can be defined as angioblasts; (2) hematopoietic cells of blood islands express TAL1, but not Flk1; (3) vasculogenesis in the embryo proper is initiated by mesoderm fated to give rise to the endocardium; (4) the maturation/morphogenesis of blood vessels can be defined in terms of a sequential pattern of expression in which TAL1 and Flk1 are expressed first followed by PECAM, CD34, VE-cadherin, and later Tie2; and (5) TAL1 expression is down-regulated in endothelial cells of mature vessels.