HATORI, M., DUTKOWSKY, J.P., SMITH, R.A., WILLIAMS, R.S., YOKOTA, T. and SAKURAI, M. Tissue Distribution of ¹²⁵I-Labeled Fetal Thyroid Hormone in Nude Mice Xenografted with Ewing Sarcoma. Tohoku J. Exp. Med., 1992, 167 (3), 239-242-Biodistribution of fetal thyroid hormone (RT₃) was studied in nude mice with Ewing's sarcoma xenografts. At 30min and 1, 2, 4, 8, and 12hr after injection, blood, tumor and normal organs were measured to determine the amount of radioactivity per gram of tissue. The amount of radioactivity in the blood of tumor-bearing mice decreased sharply from 7.64% of injected dose per gram of tissue (% ID/g) at 30min after inoculation to 0.45% ID/g at 12hr. The % of injected dose per gram for ¹²⁵I-labeled RT₃ in the tumor reached 1.92 at 1hr after injection and decreased to 0.22 at 12hr. The radiolocalization indices for tumor to other organs at 12hr ranged from 1.35 to 4.43. This study suggests increased localization of RT₃ in the Ewing's sarcoma as compared to other tissues in the nude mouse.