The determination of the colony formation in agar culture is a good method for studying the proliferation of T lymphocytes inducted by PHA. The suspension of lymphocytes separated from the blood of ten healthy volunteers was used in our experiments, PHA (100 µg/ml) was chosen as a mitogen. The kinetics and radiosensitivity of T lymphocytes in human peripheral blood were studied comparatively using the techniques of agar culture (0.3% agar) and liquid culture. The data obtained from the study on the kinetics of T lymphocytes suggested that the number of T lymphocyte colonies increased with culture time within 12 days, the peak of ,3,H-TdR incorporation into lymphocytes in liquid culture occured at 5 th day after seeding. These data also demonstrated that the kinetics of T lymphocytes cultured in two kinds of environment were differnt. The study of the radiosensitivity of T lymphocytes in agar culture and in liquid culture showed that the decrease in the number of colonies and rate of ,3,H-TdR incopration were different in the dose ranges of 0~100 rads and 100~600 rads. i. e, the declining tendency in colonies formation or in ,3,H-TdR incorporation ratio was characterized by a biphasic change. In the range of 0~100 rads, the D,0, values were 171.0 rads for colony-forming cells in agar culture and 434.0 rads for the proliferating T lymphocytes in liquid culture. In the range of 100~600 rads the D,0, values were 588.0 and 736.0 rads respectively. The foregoing results demonstrated that T lymphocytes in human peripheral blood were not homogeneous in radiosensitivity and that the colouy technique was a more sensitive method to test the proliferating capability of cells in culture.