This substance was synthesized by one of the authors and studied pharmacologically in an intensive and extensive way by the other in connection with a research instituted at the suggestion of an orthopedic surgeon, Dr. Henry Keller, of New York, in quest of a drug suitable for injection into joint cavities for facilitation of x-ray examinations. The compound is a di-sodium salt of tetra-iodo-ortho-sulpho benzoic acid having the following structural formula: It is a beautiful light yellow crystalline powder freely soluble in water, being practically odorless and tasteless. The molecular weight of this compound is 789.68 and its iodine content, 67.72%. An isotonic solution of the salt requires a concentration of 11.3%, whereas comparative studies on sodium iodide were found to produce isotonic solutions in the concentration of 2.2%. Such an isotonic solution of the new compound contains about 4 times as much iodine as an isotonic solution of sodium iodide and, in addition, is devoid of the objectionable pharmacological features of an inorganic salt solution. A 10% solution of the salt was made the starting point for all the experiments performed. Such solutions were found to be non-irritating on instillation into the conjuctival sac and application to other mucous membranes. Subcutaneous and intramuscular injections into rabbits, dogs and other animals gave no evidence of local irritation or infiltrations. Intravenous injections of the solution were well tolerated by all higher animals and quantities of 100 mg. per kilo produced no appreciably toxic effect in intact animals. The lethal dose on intravenous injection for rabbits was found to be 0.5 gm. per kilo, for cats under ether anesthesia, 0.5 gm. per kilo, and for dogs under ether anesthesia, 0.4 gm. per kilo.