The development of hematopoietic stem cell activity in the mouse embryo is a complex process that begins around day 7 of gestation. Hematopoietic stem cells (HSCs) are first detected in the yolk sac, then in the fetal liver, and finally in the adult bone marrow. Recent studies have shown that the aorta-gonad-mesonephros (AGM) region of the developing mouse embryo also possesses hematopoietic stem cell activity. This activity appears before that of the yolk sac and liver, suggesting that the AGM region may be the source of liver multilineage progenitors. However, these assays are specific for myeloid and erythroid lineages, not lymphoid. Progenitor activities in the AGM, yolk sac, and fetal liver were compared for their ability to produce in vivo macroscopic colonies on the spleen. The first statistically significant CFU-S progenitors appear at the 24 somite pair stage concurrently in the embryo body and the yolk sac. The number and frequency of CFU-S progenitors in the AGM region surpasses that of the yolk sac and peaks in the 38–40 somite pair stage of the 10 dpc embryo. This activity reaches a maximum in the AGM region just before the activity can be found in the fetal liver, suggesting that the AGM region may be the source of liver multilineage progenitors. However, these assays are specific only for the myeloid and erythroid lineages, not the lymphoid lineages. Progenitors for T lymphoid lineage cells have been found in the embryonic yolk sac beginning at 8–9 dpc using in vitro thymic organ cultures or stromal cell cocultures. In vitro cocultures of yolk sac, embryo body, and splanchnopleuric mesoderm with stromal cell lines that support the differentiation of B lymphoid cells have demonstrated B cell activity beginning as early as 8–9 dpc in these tissues. Thus, all hematopoietic lineages can be produced by early embryonic tissues. However, a common embryonic progenitor–stem cell for all these lineages cannot be examined using these assays. The most stringent and reliable measure of pluripotential hematopoietic stem cell (HSC) activity has been through the generation of adult radiation chimeras. Complete and long-term hematopoietic repopulation of adult mice has previously been demonstrated for whole and stem cell-enriched adult bone marrow and fetal liver. In addition, pluripotential and long-term HSC activity has been observed from late 11 dpc yolk sac, long term after transplantation into lethally irradiated mouse recipients. The reports of HSC activity in yolk sac at earlier times in mouse embryonic development are sparse either because the proceduresThe development of hematopoietic stem cell activity in the mouse embryo is a complex process that begins around day 7 of gestation. Hematopoietic stem cells (HSCs) are first detected in the yolk sac, then in the fetal liver, and finally in the adult bone marrow. Recent studies have shown that the aorta-gonad-mesonephros (AGM) region of the developing mouse embryo also possesses hematopoietic stem cell activity. This activity appears before that of the yolk sac and liver, suggesting that the AGM region may be the source of liver multilineage progenitors. However, these assays are specific for myeloid and erythroid lineages, not lymphoid. Progenitor activities in the AGM, yolk sac, and fetal liver were compared for their ability to produce in vivo macroscopic colonies on the spleen. The first statistically significant CFU-S progenitors appear at the 24 somite pair stage concurrently in the embryo body and the yolk sac. The number and frequency of CFU-S progenitors in the AGM region surpasses that of the yolk sac and peaks in the 38–40 somite pair stage of the 10 dpc embryo. This activity reaches a maximum in the AGM region just before the activity can be found in the fetal liver, suggesting that the AGM region may be the source of liver multilineage progenitors. However, these assays are specific only for the myeloid and erythroid lineages, not the lymphoid lineages. Progenitors for T lymphoid lineage cells have been found in the embryonic yolk sac beginning at 8–9 dpc using in vitro thymic organ cultures or stromal cell cocultures. In vitro cocultures of yolk sac, embryo body, and splanchnopleuric mesoderm with stromal cell lines that support the differentiation of B lymphoid cells have demonstrated B cell activity beginning as early as 8–9 dpc in these tissues. Thus, all hematopoietic lineages can be produced by early embryonic tissues. However, a common embryonic progenitor–stem cell for all these lineages cannot be examined using these assays. The most stringent and reliable measure of pluripotential hematopoietic stem cell (HSC) activity has been through the generation of adult radiation chimeras. Complete and long-term hematopoietic repopulation of adult mice has previously been demonstrated for whole and stem cell-enriched adult bone marrow and fetal liver. In addition, pluripotential and long-term HSC activity has been observed from late 11 dpc yolk sac, long term after transplantation into lethally irradiated mouse recipients. The reports of HSC activity in yolk sac at earlier times in mouse embryonic development are sparse either because the procedures