The study of Friend and Rauscher murine leukemia viruses has produced a variety of evidence regarding the nature of the target cell(s). These viruses produce in mice leukemias with a strong erythroid component. However, they are also pancytotic in their action, with demonstrable effects on differentiating myeloid and thromboid cells, the immuno-responsive cells, and the peripheral lymphoid cells as well. In addition, it has been noted that a variety of factors can influence disease expression, including the variety of mouse strain, the hematopoietic cell line being observed, and the tissue microenvironment in which leukemogenesis is taking place, as well as the viral substrain itself. The data available indicates that the target cells are definitely to be found among the most primitive of the hematopoietic progenitor cells of both the marrow and the spleen. However, from an analysis of this data it would appear that the virus target is not exclusively limited to a single type of hematopoietic precursor cell. Rather it is suggested that there is a closely related family of targets, consisting of the uncommitted pluripotent stem cell and the committed progenitor stem cells of the erythroid, myeloid, thromboid and immune cell lines. The evidence for each of these types of hematopoietic cells is reviewed.