Abstract To ensure the B cell differentiation stage specificity of the intronic Eμ element and of the locus control region (LCR) that lies downstream of the IgH chain locus, we generated transgenic mice harboring a V H promoter-GFP reporter gene linked to the 3′LCR region and the Eμ element. By flow cytometry, GFP + lymphocytes were observed amongst pro-B cells (B220 +CD43 +CD117 +) and at all stages of differentiation up to mature B cells (B220 +IgM +IgD +). Expression was strictly confined to cells committed to the B lymphocyte lineage as judged by the lack of GFP +Thy1,2 + cells (T lymphocytes) and GFP +B220 −CD117 +CD43 + cells (uncommitted lymphohematopoietic progenitors). Therefore, the Eμ-GFP-3′LCR transgene is not expressed by hematopoietic stem cells, begins its expression in pro-B cells and is specifically active at all stages of B cell maturation. The combination of 3′ and 5′ IgH regulatory elements thus appears as a potentially useful cassette in transgenes that require a stringent and early B lineage-specific expression.