The proliferative capacity of bone marrow cells from thymus-deprived nude mice was investigated in lethally irradiated recipients. Although the colony-forming capacity of these cells was found to be similar to that of normal littermates, a reduction in the number of nucleated cells was observed in the bone marrow of nude mice. Moreover when the radioprotective effect of such cells was studied, it was found that 5 x 105 or 2 x 106 bone marrow cells from nude mice were less effective in restoring hemopoiesis and establishing permanent chimerism than similar amounts of bone marrow cells of normal controls. In addition, irradiated animals surviving after injection of bone marrow cells from nude mice were found to have lower immune responses to SRBC than normal chimeras. The possibility that mortality of irradiated recipients injected with bone marrow of nude mice is due to the presence of a latent infective agent or of some inhibitory factor of hematopoiesis in the bone marrow of such nude mice is shown to be improbable. Alternatively it is suggested that nude mice suffer from an intrinsic defect in the proliferative capacity of their bone marrow colony-forming cells (CFUs).