A detailed study has been made of the method for labeling streptokinase with 99mTc in order to get a radioactive indicator for rapid scintigraphic visualization of thrombi and emboli. The best method found for preparing 99mTc-streptokinase was by reducing 99mTc-pertechnetate with 2 mumole SnCl2 at pH 0.7 and then adding 50,000-75,000 IU of streptokinase at a final pH of about 2. After 1 hr of equilibration the labeling efficiency was 75-80% as determined by the method of gel chromatography column scanning. The dynamic behavior and distribution of 99mTc-streptokinase in different organs was studied in rabbits where a high uptake was found in liver. In a clinical investigation using 99mTc-streptokinase prepared according to this work, however, the liver uptake in man was only 10-20%.