Obstetricians caring for expectant mothers use red cells tagged with ⁵¹Cr to find the exact location of the placenta. For example, in the condition known as _placenta previa_, the placenta—the organ within the uterus by which nourishment is transferred from the mother’s blood to that of the unborn child—may be placed in such a position that fatal bleeding can occur. A radiation-counting instrument placed over the lower abdomen gives information about the exact location of the placenta. If an abnormal situation exists, the attending physician is then alert and ready to cope with it. The advantages of chromium over iodine-131, which has also been used, are that smaller doses are required, and that there is no transfer of radioactivity to the fetal circulation. Still another common measurement using ⁵¹Cr-labeled red blood cells is the determination of the amount and location of bleeding from the gastrointestinal tract (the stomach and bowels). The amount is found by simple measurement of chromium in the blood that appears in the stools. To find the location is slightly more complicated. The intestinal contents are sampled at different levels through an inserted tube, and the radiation of the samples determined separately. Finally, gastrointestinal loss of protein can be measured with the aid of ⁵¹Cr-labeled blood serum. The serum is treated with CrCl₃ and then injected into a vein. In several very serious ailments there is serious loss of blood protein through the intestines. In these conditions the ⁵¹Cr level in the intestinal excretions is high, and this alerts the doctor to apply remedial measures. Cobalt-60 Vitamin B₁₂ is a cobalt compound. Normally the few milligrams of B₁₂ in the body are stored in the liver and released to the blood stream as needed. In _pernicious anemia_, a potentially fatal but curable disease, the B₁₂ content of the blood falls from the usual level of 300-900 micromicrograms per milliliter (ml) to 0 to 100 micromicrograms per ml. The administration of massive doses of B₁₂ is the only known remedy for this condition. If the B₁₂ is labeled with radioactive cobalt, its passage into the blood stream may be observed by several different methods. The simplest is to give the B₁₂ by mouth, and after about 8 hours study the level of cobalt radioactivity in the blood. Cobalt-60 has been used for several years, but recently cobalt-58 has been found more satisfactory. It has a half-life of 72 days while ⁶⁰Co has a 5.3-year half-life. This reduces greatly the amount of radiation to the patient’s liver by the retained radioactivity. Iodine-131 Like chromium-51, iodine is a versatile tracer element. It is used to determine blood volume, cardiac output, plasma volume, liver activity, fat metabolism, thyroid cancer metastases, brain tumors, and the size, shape, and activity of the thyroid gland.