Murine hematopoietic stem cells (HSCs) originate from mesoderm in a process that requires the transcription factor SCL/Tal1. To define steps in the commitment to blood cell fate, we compared wild-type and SCL−/− embryonic stem cell differentiation in vitro and identified CD41 (GpIIb) as the earliest surface marker missing from SCL−/− embryoid bodies (EBs). Culture of fluorescence-activated cell sorter (FACS) purified cells from EBs showed that definitive hematopoietic progenitors were highly enriched in the CD41+ fraction, whereas endothelial cells developed from CD41− cells. In the mouse embryo, expression of CD41 was detected in yolk sac blood islands and in fetal liver. In yolk sac and EBs, the panhematopoietic marker CD45 appeared in a subpopulation of CD41+ cells. However, multilineage hematopoietic colonies developed not only from CD45+CD41+ cells but also from CD45−CD41+ cells, suggesting that CD41 rather than CD45 marks the definitive culture colony-forming unit (CFU-C) at the embryonic stage. In contrast, fetal liver CFU-C was CD45+, and only a subfraction expressed CD41, demonstrating down-regulation of CD41 by the fetal liver stage. In yolk sac and EBs, CD41 was coexpressed with embryonic HSC markers c-kit and CD34. Sorting for CD41 and c-kit expression resulted in enrichment of definitive hematopoietic progenitors. Furthermore, the CD41+c-kit+ population was missing from runx1/AML1−/− EBs that lack definitive hematopoiesis. These results suggest that the expression of CD41, a candidate target gene of SCL/Tal1, and c-kit define the divergence of definitive hematopoiesis from endothelial cells during development. Although CD41 is commonly referred to as megakaryocyte–platelet integrin in adult hematopoiesis, these results implicate a wider role for CD41 during murine ontogeny.