We show by an in vitro approach that multipotent hemopoietic cells can be detected in the body of the mouse embryo between the stages of 10-25 somites (8.5-9.5 days of gestation)--i.e., prior to liver colonization (28-32 pairs of somites). Interestingly, hemopoietic cells appear in parallel in this location, the paraaortic splanchnopleura, and in the yolk sac, where they represent a new generation by reference to the primitive hemopoietic stem cells. Lymphoid cell clones, which could differentiate into mature B cells, were obtained from yolk sac and paraaortic splanchnopleura cell preparations but not from other tissues of the embryonic body. These B-cell precursors were first detected around the stage of 10 somites; thereafter, their initial minute numbers increased in parallel in the yolk sac and the paraaortic splanchnopleura, suggesting that their emergence in the two sites was simultaneous. By single cell manipulation, we show that these precursors can generate B and T lymphocytes and myeloid cells; these precursors can thus be defined as multipotent hemopoietic cells.