The colony forming capacity of bone marrow from neonatally thymectomized mice is reduced in comparison with that of normal animalsmin addition to this quantitative change, we observed that the bone marrow of thymectomized animals has a reduced radioprotective effect upon inoculation into lethally irradiated recipient mice. It was also found that the cellularity of spleen colonies derived from bone marrow of thymectomized animals is lower than that of intact controls. In vitro uptake of 3H-thymidine into cells of spleen colonies, and rate of DNA synthesis measured in vitro were found to be reduced in cells derived from bone marrow of thymectomized donors; The initially observed reduction in colony forming capactiy of bone marrow from neonatally thymectomized mice could be reversed by thymus reimplantation=