Localization of the placenta is important in the management of the obstetric patient with third trimester antepartum bleeding. At some stage of management, usually after bleeding has ceased for twenty-four to forty-eight hours, careful speculum examination may disclose local causes, and such patients can be spared further investigation. For the remainder, a number of methods of excluding placenta praevia have been proposed, as indicated in Table I. Regrettably, all of the roentgenologic procedures for placental localization are attended by disadvantages, and for that reason we have been interested in the application of radioactive isotopes to this problem. The first placental localization by means of a radioactive isotope was reported by Browne and Veall (1, 2) in 1950. Radioactive sodium (Na24) in saline solution was injected intravenously to locate anterior placentas in order to aspirate maternal placental blood for research studies. Isotopic methods of placental localization depend on the fact that the placenta contains a pool of maternal blood, which, radioactive, may be detectable. Radioactive sodium diffuses out of the blood stream so rapidly that counting has to be completed very quickly, and relatively high levels of radioactivity are needed. Human serum albumin labeled with radioactive iodine, however, is retained in the maternal blood stream. The use of this agent was first reported by Weinberg and his associates (3) in 1957, although they had suggested it a year earlier (4). Further experience using radioactive isotopes for placental localization is recorded by various authors (5–9). Method Three microcuries of radioactive iodinated (I131) human serum albumin is injected into the antecubital vein. Fifteen minutes is allowed for distribution of the material in the maternal circulation and the patient is then placed in the supine position. The fetal position and the height of the fundus above the symphysis pubis are recorded. A scintillation detector with a 1 × Linch crystal with lateral shielding and anterior collimation, but with no filter in front of the crystal, is used. It is mounted in a Nuclear Chicago model SA2 detector stand which permits rapid positioning. With the detector connected to a count rate meter (Fig. 1), the radioactivity is counted in a reference position over the precordium and in 21 positions around the detector in a horizontal position so that it can pick up radioactivity from the posterior aspect of the uterus. Readings are also taken at the midpoint of each inguinal ligament and at the symphysis pubis, with the detector directed toward the cervix. Two or more readings are made at each position and the mean values are calculated. To minimize the uptake of I131 by the thyroid gland, Lugol's solution is prescribed daily for two weeks following the test and preferably also for two days preceding it.