Abstract: A 18 mer oligonucleotides sequence is synthesized and grafted in 5′ position with a tyramine group. The tyramine derivatized oligonucleotides is labeled with ~(125)I using the chloramines-T method. The stability of ~(125)I-Tyramine-FR-ASON in blood is measured. The labeling yield, radiochemical purity and specific activity are 80.7%, 98.7% and 100 GBq/g, respectively. The radiochemical purity of ~(125)I-Tyramine-FR-ASON is over 90% at 24 h in ambient atmosphere. The polyacrylamide gel electrophoresis results show that there is not any additional band at 2, 6 and 24 h post incubation. The radioiodination method by conjugation of the oligonucleotides with tyramine is very successful.