[Quantitative determination of transplantable hemopoietic stem cells by limiting dilution method]

The concentration of hemopoiesis restoring units (HRU) in bone marrow of mice was assayed by using the limiting dilution method in transplantation to lethally irradiated mice. 7 to 12.7 HRU were found in 10(6) bone marrow cells of CBF1 mice and 19.2 to 50.6 HRU in BCT6F1 mice when the survival of the recipients was registered in 4 weeks after transplantation. The proportion of not surviving recipients increased with time when marrow doses were low (2.5 X 10(4) to 2 X 10(5) cells or 0.5-2.5 HRU per mouse) and remained stable when middle or high marrow doses (10(6)-10(7) cells) were used.