By using a spleen colony transfer technique and sex chromosome determinations as a cytogenetic marker, we compared the spleen colony-forming cells of bone marrow origin from three strains of mice, i.e., LACA, C57, and (LACA female X C57 male) F1. The proliferation potential of spleen colony-forming cells was found to be heterogeneous in nature. Some of the cells have the essential characteristics of hemopoietic stem cells (Ps-CFUS), i.e., the capacity for sustained proliferation and differentiation into myeloid and lymphoid lineages of cells, whereas others exhibit the capacity to form gross colonies on irradiated recipient spleen which are capable of differentiating different lineages of blood cells, but have lost the ability to reconstitute hemopoiesis in lethally irradiated recipients (Pg-CFUS). The results of studies on spleen colony-forming cells of F1 mice indicate the existence of a subpopulation of primitive precursors of spleen colony-forming cells (Pre-CFUs) which are not able to form spleen colonies on recipient spleens, but, under appropriate stimuli, may assume the properties of Ps-CFUS. Therefore the sequential process of blood cell differentiation may be described as follows: Pre-CFUS----Ps-CFUS----Pg-CFUS----different lineages of hemopoietic progenitors----different lineages of blood cells.