[Illustration: _Time-lapse motion pictures of the liver of a 3-year-old girl were made with the scintillation camera 1 hour after injection of 50 microcuries of iodine-131-labeled rose bengal dye. This child was born without a bile-duct system and an artificial bile duct had been created surgically. She developed symptoms that caused concern that the duct had closed. These scans show the mass of material containing the radioactive material (small light area) moving downward and to the right, indicating that the duct was still open._] For many years, a dye known as _rose bengal_ has been used in testing liver function. About 10 years ago this procedure was improved by labeling the dye with ¹³¹I. When this dye is injected into a vein it goes to the liver, which removes it from the blood stream and transfers it to the intestines to be excreted. The rate of disappearance of the dye from the blood stream is therefore a measure of the liver activity. Immediately after administration of the radioactive dye, counts are recorded, preferably continuously from several sites with shielded, collimated detectors. One counter is placed over the side of the head or the thigh to record the clearance of the dye from the blood stream. A second is placed over the liver, and a third over the abdomen to record the passage of the dye into the small intestine. Human serum albumin labeled with ¹³¹I is sometimes used for location of brain tumors. It appears that tumors alter a normal “barrier” between the brain and blood in such a manner that the labeled albumin can penetrate tumorous tissues although it would be excluded from healthy brain tissue. [Illustration: Showing:] 1 Human serum albumin labeled with ¹³¹I 2 Tumor selectively localizes labeled albumin 3 Survey of gamma radiation (Standard points) The brain behaves almost uniquely among body tissues in that a “blood-brain barrier” exists, so that substances injected into the blood stream will not pass into brain cells although they will pass readily into muscular tissue. This blood-brain barrier does not exist in brain tumors. A systematic scanning of the skull then permits location of these cancerous “hot spots”. Iron-59 Iron is a necessary constituent of red blood cells, so its radioactive form, ⁵⁹Fe, has been used frequently in measurement of the rate of formation of red cells, the lifetime of red cells, and red cell volumes. The labeling is more difficult than labeling with chromium for the same purposes, so this procedure no longer has the importance it once had.