Integrins are transmembrane receptors that play important roles as modulators of cell behaviour through their adhesion properties and the initiation of signaling cascades. The alphaIIb integrin subunit (CD41) is one of the first cell surface markers indicative of hematopoietic commitment. alphaIIb pairs exclusively with beta3 to form the alphaIIbbeta3 integrin. beta3 (CD61) also pairs with alphav (CD51) to form the alphavbeta3 integrin. The expression and putative role of these integrins during mouse hematopoietic development is as yet unknown. We show here that hematopoietic stem cells (HSCs) differentially express alphaIIbbeta3 and alphavbeta3 integrins throughout development. Whereas the first HSCs generated in the aorta at mid-gestation express both integrins, HSCs from the placenta only express alphavbeta3, and most fetal liver HSCs do not express either integrin. By using alphaIIb deficient embryos, we show that alphaIIb is not only a reliable HSC marker but it also plays an important and specific function in maintaining the HSC activity in the mouse embryonic aorta.