Mouse lymphosarcoma LS/BL cells passaged in syngeneic hosts were continuously irradiated with 60Co gamma rays at an exposure of 1R/hour. Long-term irradiation did not substantially alter the proliferative rate of leukaemic cells. Radiation exposure in the duration of 60 and 70 weeks led to an increase in the mean survival time of the hosts and in the generation time of the cells. When compared to the changes observed in the hosts of unirradiated leukaemic cells, the decrease in erythrocyte counts was greater during the first few days after inoculation of irradiated tumour cells, whereas the increase in leucocyte counts was slower.