The effect of low dose radiation on tumor metastasis in mice was investigated by using the models of B16 melanoma bloodtract pulmonary metastasis and Lewis lung carcinoma. The results were reported as follows: (1) The number of B16 melanoma pulmonary metastatic colonies of the mice pre-received different doses(25,50,75,100mGy) of whole body irradiation were lower than that of unirradiated control group mice (P