The kinetics, metabolism and radiosensitivity of T-lymphocytes in peripheral blood and colony forming unit-T lymphocyte (CFU-TL) were studied with technique of tissue culture with methycellulose and ,3,H-TdR incorporation. The linear relationship was observed between the number of CFU-TL and the planted mononucleus cells at a certain concentration of cells. The number of colonies increased with planted cells in the range of (0.5—1.5)×10,5,. It was found that in the cells cultured for 12 days,3,H—TdR incorporation still can be achieved but incorporation was lower than that in fresh separated mononucleus cells, which suggests decreased in the exposed cells. After γ-irradiation, both the number of colonies and CPM of ,3,H-TdR incorporation decreased rapidly. The result indicates that CFU-TL is more sensitive, D,0,=2.3 Gy.