Human whole blood were incubated in vitro. Lymphocytes were induced by mitogen PWM and LPS respectively. After exposure to ,60,Coγ-rays at various doses, the cells were incubated together in different combination. Lymphocyte induced by one kind of mitogen was incubated as control. The rate of lymphocyte proliferation and transformation was determinated by ,3,H-TdR incorporation. This study demonstrated that PWM-induced lymphocyte was able to enhance the stimulating effect of LPS on B lymphocyte. Lymphocytes induced by one kind of mitogen were exposed to 1-40 Gy r-rays, and the cells were incubated alone or with the lymphocyte induced by another mitogen, thus, the ,3,H-TdR incorporation became much lower than that in control. The result showed that the decrease in incorporation of ,3,H-TdR was dose dependent and it had a negative correlation. The relationship between the logarithm of radiation dosage and the radioactivity counts for ,3,H-TdR incorporation was expressed by the linear regression equations. By T-test, it was shown that PWM-induced lymphocyte was damaged more markedly than LPS-induced lymphocyte by irradiation. The stimulating effect of the PWM-induced lymphocyte disappeared after 1 Gy irradiation, but the LPS-induced lymphocyte after 1-2 Gy irradiation was still stimulated by the PWM lymphocyte. It demonstrated ionizing radiation caused defect of immunity in organism, the damage of the PWM-induced T helper lymphocyte was more important in maintaining the function of immunity.