Beschreibung:
<jats:title>Abstract</jats:title><jats:p>Lethally irradiated LAF<jats:sub>1</jats:sub> mice were protected by various numbers of isogenic bone marrow cells. At intervals, cells from femoral bone marrow and spleen were assayed for their content of colony forming units (CFU) by retransplantation into other lethally irradiated mice and counting of spleen nodules at eight days. CFU numbers in femoral marrow were restored to preirradiation levels by about eight weeks. In the spleen, restoration was earlier and more rapid and for several weeks the CFU content exceeded the normal level. Prior splenectomy did not alter the pattern of CFU restoration in the femoral marrow postirradiation. However, splenectomy did appear to slightly improve survival of irradiated marrow‐protected mice. It is argued that the CFU may not be the essential protective cell, and the existence of a precursor “stem” cell in normal marrow is postulated.</jats:p>