Abstract During mid-gestation in the mouse the fetal liver is the major hematopoietic organ. Stem cell populations from this organ have been extensively characterized by retroviral marking and transplantation into irradiated adult animals. To a large extent these cells behave identically to their adult bone marrow counterparts in that they give rise to permanent, oligoclonal hematopoietic systems. Stem cell populations have also been directly characterized and purified by a variety of immuno-chemical and physical criteria. A combination of clonal marking and purification has provided a detailed physical description of stem cells which are capable of permanent hematopoietic function in vivo. An overall stem cell enrichment of 1000- to 2000-fold has been achieved.