A rapid and efficient method for the synthesis of 125I-labeled oligodeoxynucleotides ([125I]ODNs) is described. The key intermediates are tributylstannylbenzamide-modified ODNs (Sn-ODNs). Reaction conditions are described for the preparation of 5'-modified Sn-ODNs. Treatment with NaI and chloramine T gave conversion to the desired I-ODN, which was easily isolated by reversed phase chromatography. Thermal denaturation (Tm) studies showed that hybridization properties were not disturbed by the 4-iodobenzamide modification. An [125I]ODN was prepared and characterized by hybridization to 32P-labeled DNA targets. Sequence specific cleavage of the target DNA strand by 125I was measured.