Increasing interest in the initial steps of hemopoiesis in the embryo is prompted by the lack of information about the primordial origin and the nature of hemopoietic stem cells. It appears critical to understand the emergence, diversification and differentiation potential of hemopoietic cells in the preliver embryo. In-vitro studies of B-potential in the mouse embryo prior to liver colonization show that, contrary to interpretations prevalent 10 years ago, there are two sources of B-cell progenitors (and most probably stem cells) at these stages: the yolk sac and the paraaortic splanchnopleura within the embryo proper. The analysis of the phenotype and differentiation potential of precursors from both sources shows them to be very similar, while different from the fetal liver progenitors. Indications for an independent generation of precursors in both sites are reviewed.