[en] The method of labelling bromosulphthalein (BSP) with iodine-131 by direct iodination is described. The yield of the reaction depends on the pH value, quality of Na¹³¹I used, and quantity of inactive KI added. Optimal conditions of separating BSP¹³¹I from iodine-131 have been obtained when using Amberlite IRA-400, at the elution pH=7-7.3. The products obtained were stable for 21 days if the specific activity did not exceed 0.20 mCi/mg. (author)