Since the new concept of "Radiation Hormesis" was put forward, much work has been done in this area in the world. In 1977, our staffs found that 0.1 Gy γ-ray irradiation could enhance ,3,H-TdR incorporation in lymphocytes. Other authors also observed the increasing reactivity of lymphocytes to PHA in the residents of high radiation background area. In the study, three kinds of monoclonal antibody were used to separate subsets of lymphocyte, and then the founctional changes of the separated subsets after low dose irradiation (LDI) were studied. This kind of researching paper has not been seen in publications in the world.,McAb CD4、CD8 and B were used to obtain CD4,+,、CD8,+, and B cells respectively with "Panning" method, the cells were irradiated with X-ray machine (200 kV, 10 mA) for 0, 0.02, 0.05,0.1,0.2,0.5 Gy. ,3,H-TdR incorporation was used to reflect functional changes of subsets after LDI. The results (in the table) indicated that ① three kinds of subsets could be stimulated by LDI (within 0.2 Gy). The peak effect for CD4,+, and B cells was induced by 0.1 Gy irradiation: for CD8,+, cell, the peak effect dose was 0.05 Gy; ② between 0.02 Gy and 0.2 Gy, for same dose, the stimulating effect of CD4,+, was higher than that of CD8,+,.,This result has an important significance in demonstrating the immune mechanism of radiation hormesis. The past viewpoint suggested that immune hormesis was caused by the damage of radiosensitive T cell (Ts) after LDI. Recently, some authors proved that no change of the ratio of Ts to T,H, existed after LDI. In our study, we found the values of ,3,H-TdR incorporation in CD4,+, was bigger than that in CD8,+, after LDI. Obvious stimulating effect could still be observed after 0.2 Gy irradiation, it indicated that subsets separated by McAb could have a wide stimulating dose range for LDI.