PU.1 is a member of the ETS family of transcription factors and is required for the development of multiple hematopoietic lineages. PU.1-/- mice die from hematopoietic failure at about embryonic day 18.5 (e18.5) and show a complete absence of B cells, mature T cells, and macrophages. This phenotype suggests that PU.1 may function at the level of the hematopoietic stem cell (HSC) or a multilineage progenitor. To investigate the role of PU.1 in the regulation of HSCs, PU.1-/- embryos were analyzed at various stages of embryonic development. The absolute number and frequency of HSCs were determined by flow cytometric analysis of c-Kit+Thy-1.1 loLin-Sca-1+ (KTLS) cells. We found that KTLS cells were absent or severely reduced in PU.1-/- fetal liver from e12.5 to e15.5. Progenitor cells with a c-Kit+Lin-AA4.1+ and c-Kit+Lin-CD34+ phenotype were also severely reduced. In addition, PU.1-/- fetal liver at e14.5 lacked common myeloid progenitors (CMPs) and granulocyte-macrophage progenitors (GMPs) but retained megakaryocyte-erythroid progenitors (MEPs). Consistent with the loss of HSC activity, a 10-fold reduction in erythroid progenitors (mature erythroid burst-forming units [BFU-Es]) was observed between e14.5 and e16.5. These data suggest that PU.1 plays an important role in the maintenance or expansion of HSC number in murine fetal liver. © 2004 by The American Society of Hematology.