Effects of Juzen-taiho-toh (TJ-48) on the recovery of hemopoietic systems from radiation injury are analyzed. Female C57BL/6N mice (6-8 weeks old) were irradiated at doses of 1, 2, 3, 5, or 7 Gy from a 60Co source. After irradiation, the mice were given TJ-48 (1.25 g in 100 ml drinking water). Seven days after irradiation, the mice were sacrificed, and bone marrow (both femurs), thymus, spleen, and peripheral blood counts were made. The bone marrow cells were used for fibroblast colony-forming unit (CFU-f), spleen colony-forming unit (CFU-S), granulocyte-macrophage colony-forming unit (CFU-GM), erythroid colony-forming unit (CFU-E), and erythroid burst-forming unit (BFU-E) assays. No difference was observed between the experimental and control groups except for CFU-S counts. In the assay for day-14 CFU-S, the mice injected with TJ-48-treated bone marrow cells showed better general condition (including increased body weight) and heavier spleens with larger and more numerous colonies. The effect of TJ-48 does not seem to be elicited via the hemopoietic microenvironment, because mice that had been given TJ-48 before irradiation at 8 Gy and then injected with syngeneic bone marrow cells did not show enhanced day-14 CFU-S counts. These results suggest that TJ-48 manifests a radioprotective effect by increasing the number and size of day-14 CFU-S.