Suspensions of cells in the bone marrow and in the spleen of normal mice were prepared. Two groups of lethally irradiated mice were given cells from bone marrow and two groups of lethally irradiated mice were given cells from the spleen of the same donor mouse. One of the groups injected with bone marrow and one injected with spleen cells were killed at 9 days for determination of CFU. The remaining animals were followed for 100 days to determine the mortality statistics. The mortality among mice not given cells was 100 per cent; among mice injected with cells, the mortality was reduced. The data permit a comparison of the reduction in mortality (protection) following injection of bone marrow CFU with protection following injection of spleen CFU. Protection with CFU of bone marrow was different from protection with CFU of spleen. It seems possible that both hemopoietic pluripotent "stem cells" and "early differentiated progenitors" can form spleen colonies and that the ratio of these CFU may be different for bone marrow and spleen.