Summary: Precursors of hematopoietic stem cells (pre-HSCs) have been identified as intermediate precursors during the maturation process from hemogenic endothelial cells to HSCs in the aorta-gonad-mesonephros (AGM) region of the mouse embryo at embryonic day 10.5. Although pre-HSCs acquire an efficient adult-repopulating ability after ex vivo co-culture, their native hematopoietic capacity remains unknown. Here, we employed direct transplantation assays of CD45−VE-cadherin(VC)+KIT+(V+K+) cells (containing pre-HSCs) into immunodeficient neonatal mice that permit engraftment of embryonic hematopoietic precursors. We found that freshly isolated V+K+ cells exhibited significantly greater B-1 lymphocyte-biased repopulating capacity than multilineage repopulating capacity. Additionally, B cell colony-forming assays demonstrated the predominant B-1 progenitor colony-forming ability of these cells; however, increased B-2 progenitor colony-forming ability emerged after co-culture with Akt-expressing AGM endothelial cells, conditions that support pre-HSC maturation into HSCs. Our studies revealed an unexpected B-1 lymphocyte bias of the V+K+ population and acquisition of B-2 potential during commitment to the HSC fate. : Yoshimoto and colleagues demonstrate an unexpected inherent B-1-biased repopulation ability of E10.5 pre-HSCs in immunodeficient neonatal mice. The B-1-biased pre-HSCs acquire B-2 potential following ex vivo co-culture, reflecting the developmental process of hemogenic endothelial cells to HSCs through B-1-biased state in vivo. Notably, the E10.5 pre-HSCs are the first B-1a repopulating cells upon direct transplantation without co-culture. Keywords: mouse embryos, hematopoietic stem cells (HSCs), pre-HSC, AGM, B-1a cells, hemogenic endothelial cells