After whole body exposed to 7.5 cGy X-ray, the C57 BL/6 mice were I. V. injected with ,125,IUdR labelled B16 melanoma cells, the proportion of surviving labelled cells retained in lungs was detected at 24 hours after injection,and found that the surviving cells in the lungs of irradiated mice were significantly decreased. Meanwhile, the immunofunctions were tested at 24 hours after irradiation, the results showed that the number of nucleate cells and NK cytotoxity in spleens, and the phagocytotic function of macrophages in vivo were increased obviously for the irradiated mice. The results suggested that the enhancement of NK cytotoxity and macrophage phagocytotic function might be one of the important reasons for low dose radiation accelerating the clearance of tumor cells from the lungs.