Retinoids are known to have potent effects on hemopoietic stem cell integrity, and our objective was to learn whether they influence cells destined to replenish the immune system. Total CD19+ B lineage cells increased substantially in the marrow and spleens of all-trans retinoic acid (ATRA)-treated C57BL6 mice, while lymphoid progenitors were reduced. All B lymphoid progenitors were targets of ATRA in culture and overall cell yields declined without reductions in proliferation. Remarkably, ATRA shortened the time required for primitive progenitors to generate CD19+ cells. PCR analysis and a panel of retinoid acid receptor (RAR)/retinoid X receptor agonist treatments suggested that RARα mediates these responses. The transcription factors EBF1 and Pax-5 were elevated during treatment and ATRA had similar effects on human B cell differentiation. That is, it inhibited the expansion of human progenitor cells and accelerated their differentiation to B lineage cells. There may be previously unsuspected side effects of ATRA therapy, and the new findings suggest retinoids can normally contribute to the lymphopoietic environment in bone marrow. Copyright © 2007 by The American Association of Immunologists, Inc.