[en] The labelling of uracil (C₄H₄N₂O₂) with radioiodine (¹³¹I) have been carried out by means of reaction with iodomonochloride (¹³¹I). The analysis of the resulting iodinated compound was done using Whatman I paper chromatography and Silica Gel 60 F₂₅₄ thin-layer chromatography with a mixture of n-butanol-glacial acetic acid-water (75:30:50, v/v) as the eluting solvent. The result of UV spectrophotometric analysis strengthened the supposition that the labelling mechanism is an electrophilic substitution which gives 5-radioiodouracil as the resulting iodinated compound. The yield with the best labelling conditions was more than 97% so that purification of the iodinated compound was not necessary. It was observed that the final products were radiochemically stable after 8 days under ordinary storage conditions. (author). 8 refs