ABSTRACT
By using single spleen colony transplantation technique and sex chromosome typing as natural cytogenetic markers, we have been able to demonstrate that most spleen colony forming cells in the adult bone marrow or in foetal liver of inbred LACA or C57 mice possess the potential of re-establishment of myeloid and lymphoid lines of cells in lethally irradiated mice. However, most of the spleen colony forming cells in the peripheral [blood of normal mice possess weak or no potentiality to proliferate or re-establish hemopoiesis in lethally irradiated mice. Our results support the notion that the CFU-S population in mice is heterogeneous.From the nature of colony forming cells in the peripheral blood of normal mice, we are convinced that the assessment of CFU-S content relying upon the spleen colonies as the unique sign to indicate the self-renewal ability of a spleen colony forming cell is by no means valid inasmuch as some plu-ripotent . hemopoietic progenitors derived from pluripotent hemopoietic stem cells may have the same ability to form colonies in vivo.