Potassium 7,8-dicarba-nido-undecaborate was thiocyanated by nondiaphragm electrolysis. The salt Me4N+[9-SCN-7,8-C2B9H11]− was isolated and converted into the trimethylammonium salt using ion-exchange. Methods for the synthesis of water-soluble sodium 7,8-dicarba-nido-undecaborate and its biphasic iodination (for 131I introduction) were developed. Thus, the synthesis and analysis of the radioactive iodine-labeled sodium salt of 11-(131I)-9-thiocyanato-7,8-dicarba-nido-undecaborate were proposed and carried out. The toxicity of the synthesized compound was evaluated. It was established that 100 and 75 mg/kg doses are 100% lethal for test animals while no significant disorders were observed at doses of 35 and 20 mg/kg. The biodistribution of the compound in organs and tissues was studied in mice with melanoma B-16. The maximum accumulation of labeled compound in tumor, skin, muscle, liver, kidneys, and spleen was observed 3 - 6 h after administration. A comparison of drug accumulation in tumor and normal tissues showed that the preparation does not possess pronounced tumor-targeting properties.