We used a sensitive in vitro culture system to follow, in embryonic blood, the number and state of commitment of B cell precursors along ontogeny. We describe a wave of circulating multipotent progenitors, first detectable at day 10 of gestation, and reaching a maximum in absolute numbers at day 12. They are undetectable by day 14 of gestation, when committed B cell precursors can be detected in fetal liver. Embryonic marrow contains B cell progenitors by day 15. We propose that fetal liver, thymus, and bone marrow are colonized by the same wave of multipotent hematopoietic cells and define embryonic blood at day 11 of gestation as an important source of multipotent hematopoietic cells, virtually deprived of committed and mature contaminants.