To study immunologic mechanism of the suppressive effect of low dose radiation (LDR) on thymic lymphoma (TL) induced by high dose radiation (HDR) . We adopted the model that C57BL/6J mice were administered whole body irradiation with 1.75Gy X—rays one time every week for 4 weeks to induce TL. It was examined that splenic NK cytotoxic activity, IL—2 and ,γ,—IFN secretion activity, peritoneal macrophagic phagocytosis and its TNF—α secretion activity in mice with different dose 1 month after irradiation. The results showed that all the immunologic functions mentioned above in mice given 75mGy 12h before 1.75Gy every time were higher than that in mice given only 1.75Gy, and approached to the sham —irradiation mice. It suggested that the suppressive effect of LDR on TL induced by HDR may be related to the adaptive response induced by LDR and decreasing immunological functions damage caused by HDR.