Abstract
Results of experiments are reported which indicate that colony-forming units (CFU) regenerate more rapidly when transplanted into the marrow of lethally irradiated hosts pretreated with cyclophosphamide (CY) than when transplanted into those pretreated with saline. This effect is probably unrelated to the immunosuppressive properties of CY because it occurs in isogeneic strains as well as in randomly bred CF1 mice. It also does not result from an increase in the radioresistance of cells which survive after injection of CY. The data presented here are compatible with the concept that CY somehow improves the environment for hematopoietic stem cell (HSC) proliferation and thereby causes an increased rate of regeneration of transplanted HSC in the marrow cavity of the host. The possibility that this is caused by release of HSC from an inhibitory effect of cell-cell interactions on proliferation or by the release of a humoral stimulus to HSC regeneration is discussed.
