Recent studies concerning B cell development in the mouse embryo have indicated that the first cells committed to the B cell lineage arise in the para-aortic splanchnopleura at a time prior to the hematopoietic colonisation of the fetal liver [1]. The B lymphoid potential of such cells was revealed by in vivo transfer experiments, and has recently been confirmed by analysis in vitro [2]. Whether and in what way the mature B cell progeny of such progenitors contribute to the B cell repertoire in the developing embryo is less clear. This also raises the question of the quantitative and qualitative contribution of splanchnopleura and fetal liver-derived cells to the pool of developing B cells.