Disaggregated embryonic yolk sac cells and circulating peripheral blood cells were obtained from normal murine day 9 embryos, prior to the formation of the fetal liver. These cells were microinjected transplacentally into days 11-15 W mutant anemic fetuses, when the fetal liver was the major hemopoietic organ. In a small proportion of the recipient animals examined after birth, long-term repopulation by the embryonic donor hemopoietic cells was observed. The donor hemopoietic stem cells proliferated and differentiated in the hosts as evidenced by the presence of donor hemoglobins in the growing recipient host animals. Some mothers of the pups were also repopulated by the donor stem cells. These results provide direct evidence that, during early murine embryogenesis, there are functional hemopoietic stem cells which are capable of colonizing the adult hemopoietic organs and probably the fetal liver and spleen to initiate hemopoiesis in these tissues.