Radioactive iodine-labeled rabbit antibodies to beta 1-MA antigen (the marker of human colon cancer) were injected iv to BALB/c mice as well as to nude mice with xenografts of various human tumors. Preparations of iodinated antibodies contained an admixture of protein-unbound radionuclide 2-4% of total radioactivity. Part of the recipient animals were given per os 0.1% solution of KI to block the thyroid from radioactive iodine. A comparative study was made of the distribution of radioactivity in the tissues of recipients in the time course of 7 days. The thyroid of nude mice with tumors was shown to uptake much more radioactivity than that of immunologically intact mice. In addition to its protective effect of the thyroid from radioactive iodine, KI also enhanced the excretion of the preparation of labeled antibodies. Possible mechanisms of the revealed phenomena were analyzed.