Abstract
Injections of ACTH of prolonged action (twice 1.5 units per mouse) given to lethally (800 R) irradiated mice with a hind limb shielded to the middle of the tibia brought about a twofold decrease in the number of spleen colonies. ACTH injections after sublethal whole-body irradiation (600 R) did not affect the number of endogenous spleen colonies. ACTH injection of normal mice brought about a substantial decrease in the CFU content in the circulating blood at the time of increased 11-hydroxycorticosteroid concentrations in the plasma. The results obtained are interpreted as inhibition of CFU migration from a shielded area of bone marrow induced by a high 11-hydroxycorticosteroid content in the plasma that followed ACTH injections which had no mitostatic effect on CFU proliferation.
