By means of liquid culture and agar culture in vitro, the variety of ability of reproduction of human T and B lymphocytes induced by radiation has been studied comparatively. Human blood exposed to ,60,Co γ-rays in vitro was incubated respectively in Eagle's medium or agar medium. In one group of the experiment Con A was a mitogen for T lymphocytes. LPS and DS were added into the anather group of the experiments for reproducing of B lymphocytes. ,14,C-valine was incorprated into all samples of liquid cultures. The results from the comparative study showed that the ability of T lymphocytes declimed more significantly than B lymphocytes. Hence it may be suggested that T lymphocytes were more radiosensitive than B lymphocytes.