Treatment of whole-body gamma-irradiated mice with a preparation of Lactobacillus casei (LC 9018) immediately after irradiation caused a sustained increase in serum colony-stimulating activity which was followed by an enhanced repopulation of granulocyte-macrophage colony-forming cells in the femoral marrow and spleen. Numbers of blood leukocytes, erythrocytes, and platelets were increased earlier in the treated mice than in the controls, and the survival rate was elevated significantly. The radioprotective effect was dependent on the dose of LC 9018 as well as on the dose of radiation. These results demonstrate the value of LC 9018 for the treatment of myelosuppression after radiotherapy or radiation accidents.