Abstract: The exhibit shows that colloidal aggregates (10 to 20 mu) of human serum albumin I131 may be used safely by intravenous injection to perform photoscans of the heart, liver, spleen, stomach, and salivary glands in man. Large particle size suspensions (10 to 50 mu) of the same material are being investigated experimentally in animals for scanning the lungs after intravenous injection and the brain following injection into an internal carotid artery. The advantages of this test material are the relatively low radiation exposure to the target organs and the number of organs that may be examined. Radiation exposure is low because of the rapid turnover in the target organs and removal from the body, mainly by urinary excretion, within 72 hours. The mechanism of liver-spleen localization with this organic colloid is the same as for inorganic colloidal radiogold198, namely, rapid removal from the blood by the phagocytic cells of the liver and spleen. However, in contrast to the inorganic colloid, which remains in the phagocytic cells permanently, albumin is digested by proteolytic enzymes and the I131 abel is set free to re-enter the general circulation. With the thyroid blocked, the I131 is excreted mainly in the urine as free …