The action of leukemoid blood in treatment of radiation lethality was studied. Lethally X-irradiated mice were injected with leukocytes ( 100 X 10[degree] cells) of leukemoid blood. At various intervals after irradiation, the hemoglobin of surviving mice was electophoresed in starch gel. The hemoglobin pattern of these mice was characteristic of the donors from which the leukemoid leukocytes were obtained, showing that there are cells in leukemoid blood capable of transplanting and giving rise to mature erythrocytes of donor type in lethally irradiated mice. Autoradiographic studies showed that some nucleated cells of leukemoid blood can incorporate H3-thymidine in vitro.