A fitted equation with least square method to describe the retention of ¹³⁴Cs in whole body is obtained by a whole body counter. That is R(t)=18.04 exp(-9.3175t) + 45.13exp(-0.0423t), where R(t) is in %, and t in d. The equation consists of two half-life components, the fast component is T_1/2=0.07d, and the slow is T_1/2=16.14d. Study on the localization of ¹³⁴Cs at cellular level was carried out by freezing microautoradiography. The results indicate that ¹³⁴Cs was chiefly in ionizing form accumulated in red as well as white blood cells. In bone marrow cells ¹³⁴Cs predominantly deposited in young cells and less in mature cells. Distribution of ¹³⁴Cs in soft tissues such as muscle, liver and cerebrum was observed. It should be noted that ¹³⁴Cs penetrated quickly into the tissue cells. The observation of investigating radioimmunotoxicological effect induced by ¹³⁴Cs shows that the inhibition of thymocytes is higher than bone marrow cells, the spleen T lymphocytes are more sensitive to ¹³⁴Cs than B lymphocytes and lymphocytes of peripheral immune cells are more sensitive to radiation than central immune cells.