When CBA/H mice were irradiated with 600 R, and injected with 5× 107 <tex-math>${\rm CBA}/{\rm HT}_{6}{\rm T}_{6}$</tex-math> thymus cells, donor thymus cells comprised nearly 100% of the dividing population in the host lymph nodes and spleen, and more than 70% in the bone marrow on Day 5, respectively. The percentages of dividing donor thymus cells quickly declined in the bone marrow and spleen, reaching less than 5% by Day 20. When CBA/H mice were irradiated with 600 R, and injected with (a) 1× 107 CBA/H bone marrow cells plus 5× 107 <tex-math>${\rm CBA}/{\rm HT}_{6}{\rm T}_{6}$</tex-math> thymus cells, or (b) 1× 107 <tex-math>${\rm CBA}/{\rm HT}_{6}{\rm T}_{6}$</tex-math> marrow cells plus 5× 107 CBA/H thymus cells, donor marrow cells comprised more than 50% of the dividing cell population in the bone marrow and spleen on Day 5, nearly 100% by Day 20. The division of donor thymus cells in the bone marrow was totally suppressed after the injection of marrow and thymus cells. The numbers of dividing cells of donor thymus origin were far smaller in the bone marrow and spleen of mice injected with marrow and thymus cells than in those of mice injected with thymus cells. The time courses of the percentages of dividing donor thymus cells in the thymus were hardly influenced by the simultaneous injection of marrow cells. In conclusion, the thymus has cells capable of repopulating the bone marrow and spleen of irradiated host mice, but the proliferation of donor thymus cells in the bone marrow and spleen is suppressed if marrow cells are injected together with thymus cells.

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