Abstract: For more than a decade it was believed that hematopoietic stem cells express CD34. However, this dogma was recently challenged by the observation that stem cells of normal adult mice are CD34−. In order to clarify the controversy, we carried out systematic examination of stem cells by using C57BL/6 mice that are congenic for Ly-5. As reported previously, stem cells in the normal adult mice were CD34−. However, stem cells stimulated in vivo by 5-flourouracil injection or in vitro by a combination of interleukin-11 and steel factor were CD34+. The activated CD34+ stem cells reverted to CD34− when the recipients' marrow achieved steady state. The majority of G-CSF-mobilized stem cells also were CD34+ and reverted to CD34− under steady-state conditions. Most recently, we examined the developmental changes of stem cell CD34 expression. In order to gain information on the total population of stem cells we prepared CD34+ and CD34− populations of mononuclear cells without prior enrichment and studied their engrafting potentials. All stem cells from perinatal to 5-week-old mice were CD34+. In 7-week-old mice CD34− stem cells began to emerge, and the majority of the stem cells were CD34− in the 10- and 20-week-old mice. An estimated 20% of the adult stem cells expressed CD34. These observations provide insight into the current controversy regarding CD34 expression by adult hematopoietic stem cells.