Mouse lymphosarcoma LS/BL cells growing as an ascites tumor in the peritoneal cavity of C57BL mice were continuously irradiated in vivo at a low exposure rate of 1.2 Gy per day (5 rad/hr). The growth of the ascites tumor evaluated by direct counting of the cells in the peritoneal cavity and their capacity to form colonies in livers declined with increasing time of continuous irradiation. The radiosensitivity and repair ability of LS/BL cells were studied by a serial dilution method using host survival time as the end point and by the liver colony assay. The radiosensitivity of continuously irradiated LS/BL-CI cells showed no remarkable change as measured by the Do values, but from the 150th week of irradiation the initial shoulder on the survival curves appeared and its width increased with time of exposure. The extrapolation number (n) increased from 1.0 to 8.4 after 350 weeks of irradiation. The reappearance of the initial shoulder was proved with the split-dose technique.