Recent studies by Dorshkind, Yoder, and colleagues show that embryonic (E9) B-cell progenitors located in the yolk sac and intraembryonic hemogenic endothelium before the initiation of circulation give rise to B-1 and marginal zone B cells but do not give rise to B-2 cells. In studies here, we confirm and extend these findings by showing that distinct progenitors for B-1 and B-2 cells are present in the adult spleen. Furthermore, we show that the splenic B-cell progenitor population (lin – CD19 + /B220 lo/− /CD43 – ) that gives rise to B-1 cells is likely to be heterogeneous because, in some recipients, it also gives rise to B cells expressing the marginal zone phenotype (B220 hi IgM hi IgD lo CD21 hi ) and to some (CD19 – CD5 hi ) T cells. In addition to the well-known function differences between B-1 and B-2, our studies demonstrate that substantial developmental differences separate these B-cell lineages. Thus, consistent with the known dependence of B-2 development on IL-7, all B-2 progenitors express IL-7R. However, >30% of the B-1 progenitors do not express this marker, enabling the known IL-7 independent development of B-1 cells in IL-7 −/− mice. In addition, marker expression on cells in the early stages of the B-2 development pathway (CD19 – /c-Kit lo/− /Sca-1 lo/− ) in adult bone marrow distinguish it from the early stages of B-1 development (CD19 hi /c-Kit + /Sca-1 + ), which occur constitutively in neonates. In adults, in vivo inflammatory stimulation (LPS) triggers B-1 progenitors in spleen to expand and initiate development along this B-1 developmental pathway.