DEFINITIVE erythropoiesis in birds originates from stem cells that emerge in the splanchnopleural mesoderm near the embryonic aorta^1–4. The yolk sac is still generally held to be the unique provider of haematopoietic stem cells during mammalian ontogeny⁵, although there may be an alternative intraembryonic source of stem cells in the mouse fetus^6,7. Here we search for a possible non-yolk-sac source of stem cells by grafting intraembryonic splanchnopleura from 10- to 18-somite mouse embryos into adult immunodeficient SCID mice. We find significant amounts of donor-derived serum IgM, normal numbers of IgM-secreting plasma cells, and the Bla (IgM^a_brightB220_dullCD5⁺) cell subset to be fully reconstituted by donor progenitors 3 to 6 months after engraftment. The haematogenic capacity revealed in our experiments is present in a previously unrecognized site, the earliest described in the embryo, 12 hours before fetal liver colonization.