In this paper, relative effects of LDI on NK activity of blood lymphocytes in normal subjects and cancer patients in vitro and effects of LDI on K,562, cells were studied. NK activity was determined by ,3,H-TdR release assay. 21 patients and 10 normal individuals blood werd analysed. All samples were irradiated by ,226,Ra γ-rays with doses 0,5, 10, 30, 50, 80, 120cGy separately. The results indicated that the NK activity of lymphocytes in cancer patient's blood increased significantly in 50cGy irradiation group (120.5% of control). No significat changes were found in other dose groups. While in normal subjects, NK activity increased at both dose groups, 10 and 50 cGy. The peak value of NK activity appeared at dose of 50 cGy. The tendency of the increase of NK activity in normal subjects after LDI was higher than that in patients. There was significant difference of NK activity changes between two groups of persons after 50 cGy irradiation,3,H-TdR incorporation was used to study the effects of LDI on K,562, cells. The result showed that ,3,H-TdR incorporation in K,562, cells increased significantly after 10 cGy irradiation. It indicated stimulating effect. As the dose is above 80 cGy,3,H-TdR incorporation decreased significantly, which indicated that the stimulating effect disappeared.