Co-incubation of rat liver slices or homogenates with rat thyroid lobes in a Krebs-Ringer phosphate medium containing I131 resulted in inhibition of thyroidal clearance of the isotope. Substitution of rat serum for Krebs-Ringer phosphate as the incubation medium also resulted in inhibition of thyroidal I131 clearance, but the suppression was not as great as that caused by the liver slices. The inhibitory activity of liver was heat-stable, dialyzable, ethanol-soluble, Norite-nonadsorbable and resistant to hydrolysis in 1.5N NaOH. The inhibitory activity was destroyed by ashing or hydrolysis in 1.5N HCl. Inhibition of clearance also was produced by slices of kidney cortex, heart, spleen and brain, but the inhibitory activity was lost during the preparation of extracts of these tissues by methods shown to preserve such activity in liver extracts.