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Summary: It is generally considered that mouse embryonic stem cell (ESC) differentiation into blood cells in vitro recapitulates yolk sac (YS) hematopoiesis. As such, similar to YS-derived B-progenitors, we demonstrate here that ESC-derived B-progenitors differentiate into B-1 and marginal zone B cells, but not B-2 cells in immunodeficient mice after transplantation. ESC-derived B-1 cells were maintained in the recipients for more than 6 months, secreting natural IgM antibodies in vivo. Gene expression profiling displayed a close relationship between ESC- and YS-derived B-1 progenitors. Because there are no hematopoietic stem cells (HSCs) detectable in our ESC differentiation culture, successful long-term engraftment of ESC-derived functional B-1 cells supports the presence of HSC-independent B-1 cell development. : Yoshimoto and colleagues demonstrate that mouse ESC-derived B-progenitors engraft in the immunodeficient mice long-term without gene manipulation as IgM-secreting B-1 and MZ B cells, but not conventional B-2 cells. These ESC-derived B-progenitors are molecularly similar to YS-derived B-progenitors. Since no HSCs are produced in their ESC differentiation system, this result supports the emerging paradigm-shifting concept of HSC-independent lymphopoiesis. Keywords: ESCs, HSC-independent lymphopoiesis, B-1 cells, yolk sac, mouse embryo