The effect of ,60,Co γ-rays on lymphocytes induced by Con A and LPS and the relationship between these two groups of cells were investigated by means of ,3,H-TdR incorporation. The study showed that in vitro, Con A cells were able to promote the inducing effect of LPS to B cells. When Con A cells were irradiated by 10 Gy γ-rays, the ,3,H-TdR incorporation value reduced significantly and the stimulating effect of Con A cells on LPS cells disappeared. Having been irradiated by γ-rays, LPS cells were not able to be stimulated by normal Con A cells. When the groups of cells were incubated together after irradiation, the synergistic function disappeared, furthermore the suppressive effect of Con A cells on LPS cells emerged. When these two groups of cells were investigated by means of agar culture, the suppressive effect of 10Gy γ-rays on lymphocytes colony formation was more obvious Tests on 8 patients who were suffering from carcinoma of nasopharynx showed that after a course of treatment with ,60,Co γ-rays, the incorporation value in Con A cells became much smaller, the stimulating effect of ConA cells on LPS cells disappeared. LPS cells could not be stimulated by normal Con A cells. The study demonstrated that the radiosensitivity of Con A cells was higher than that of LPS cells.