In designing functional assays for the various classes of hematopoietic cells described in this book, one needs to consider the properties of the cell to be measured which must be incorporated into the assay design, and the end points to allow its specific detection. The most primitive hematopoietic stem cells (HSC) in mouse and man are characterized by two cardinal properties that distinguish them from more mature clonogenic cells and their terminally differentiated progeny. Firstly, HSCs are pluripotent: they are characterized by the potential to differentiate into all of the eight major lineages of lymphoid, myeloid, and erythroid cells in vivo (1-3). Secondly, HSCs are able to self-renew, or generate daughter stem cells in vivo and in vitro that are functionally identical to the stem cell that gave rise to them (3-5). These hallmark properties of HSCs are measured empirically by their potential to regenerate and maintain lymphocytes, granulocytes, and erythrocytes upon transplantation into lethally irradiated or immunocompromised primary and secondary hosts. However, functional assays for primitive HSCs must also consider the fact that differentiated cells present in the hematopoietic organs at different times after bone marrow transplantation are derived from different types of precursors (6), and particularly at later times, cannot be assumed to be of donor origin (7). Support for this concept derives from the relatively recent demonstration in mice that most, if not all, spleen colonies detectable ≈ 2 wk after transplantation originate.