Human lymphocytes in heparinized blood were collected through a tetoron fiber column. About 5×105 lymphocytes in a milliliter of Eagle's essential moiium containing 20% autoserum were cultured with phytohemagglutinin (PHA) and exposed to x-irradiation. The results were as follows: (1) Morphological blast formation was not affected by x-rays below 300R. About 60% of lymphocytes could transform even after an exposure to 2, 000 R. (2) Autoradiography after continuous labeling with 3H-thymidine proved that entering of the lymphocytes into the DNA synthetic phase was suppressed by radiation. The percentage of cells entering the first DNA synthesis was dependent on the radiation dose; 96, 92, 73, 48, 37, 42 and 36% after the irradiation of 0, 150, 300, 500, 1, 000, 1, 500 and 2, 000R, respectively, 2 to 3 hours after the addition of PHA. (3) Accumulation of mitosis by coiehicine was prevented by irradiation depending on doses; 72, 36, 37, 8, 8 and 5% of original lymphocytes entered the first mitosis after the irradiation of 0, 140, 300, 500, 1, 000 and 2, 000R, respectively, at 2 to 3 hours culture. At about 2, 000R, not more than 10% of stimulated lymphocytes entered mitosis in 5 days' incubation.