To determine if mononuclear cells proliferating in murine hemopoietic spleen colonies were pluripotential in addition to possessing kinetic features of stem cells, we performed sequential studies of mice during their recovery from a split-dose irradiation regimen of 850 roentgens leg shielded-3-hr interval-850 roentgens leg irradiated (850R L.S. 3- L.I.). Injecting tritiated thymidine during stem cell compartment repletion 3 and 4 days after 850R L.S. 3- L.I. resulted in heavily labeled mononuclear cells resembling medium to large leptochromatic lymphocytes in the portion of spleen removed an hour after injection. The splenic remnant obtained from the same mouse 24–48 hr later contained lightly labeled erythroblasts, myeloid cells, and lymphoid cells. Grain counts suggested that erythroblasts and their precursors had undergone about four divisions, myeloid cells and their precursors two to three divisions, and lymphoid cells and their precursors two to three divisions during the 48-hr period. Similar studies in plethoric mice demonstrated the labeling of mononuclear cells on day 4 and their differentiation to myeloid and lymphoid cells by day 6. This finding confirmed that the labeled mononuclear cells were not exclusively erythroblast progenitors. On the basis of these and previous studies of post-irradiation survival and erythropoietic recovery, we conclude that these endogenous monomuclear cells, which resemble medium to large leptochromatic lymphocytes and replicate during stem cell compartment repletion, are pluripotential hemopoietic stem cells.