A stromal cell clone, ST2, which can support both myelopoiesis and B lymphopoiesis of adult bone marrow was used as an in vitro microenvironment for investigating the ontogeny of the B cell progenitor in murine embryos. The B cell progenitor clonable on an ST2 layer first become detectable in the embryonal body rather than in the yolk sac around day 9.5 of gestation. As soon as it develops in the embryo, it enters the blood circulation and becomes detectable both in the developing fetal liver and the yolk sac of the 10 day embryo. On the other hand, mast cell and polymorphonuclear cell progenitors, which are also generated on the ST2 layer, develop first in the yolk sac and migrate to the fetal liver around day 10-11 of gestation. At the late stage of embryonal development, day 15-16 of gestation, the B cell progenitor enters the femur as vascularization of the femur starts. These results suggest that the localization of the committed stem cells for various hemopoietic cell lineages differs in the early embryo, although the localization of the pluripotent stem cells is yet to be determined.