The results of experiments using 131I-labelled monoclonal antibody (131I-MoAb) were described. The experiments were staged on nude male mice with transplanted human colon adenocarcinoma of the 34th generation (RTK-12). Tumor sizes by the time of 131I-MoAb administration varied from 1 to 3 cm. 131I-MoAb specific radioactivity was 456.6 kBq/micrograms in the volume of 0.3 ml. The same amount of 131I-Na was administered to control animals. Scanning on the Scintimat-2F (Siemens) was performed 1, 2, 3 and 4 days after injection of 131I-MoAb. Three days before scanning 0.2% KI was added to the drinking water. Some animals were decapitated on the 2nd-3rd day for radiometry of organs and tissues. After the injection of 131I-MoAb 10-fold less than the CEA serum concentration and with specific radioactivity of 370-444 kBq its tumor accumulation in 48 h was 10-15-fold higher than that in the surrounding tissues resulting in better tumor scans. Intensive dehalogenation of 131I-MoAb in vivo and related 131I accumulation in the thyroid and adrenal glands (in the medullary layer) required the administration of stable iodine not only before but also after injection of the radioactive agent.