A recently reported selective agonist of the human A3 adenosine receptor (hA3AR), MRS5127, (1′R,2′R,3′S,4′R,5′S)-4′-[2-chloro-6-(3-iodobenzylamino)-purine]-2′,3′-O-dihydroxy-bicyclo-[3.1.0]hexane, was radioiodinated and characterized pharmacologically. It contains a rigid bicyclic ring system in place of a 5′-truncated ribose moiety, and was selected for radiolabeling due to its nanomolar binding affinity at both human and rat A3ARs. The radioiodination of the N6-3-iodobenzyl substituent by iododestannylation of a 3- (trimethylstannyl)benzyl precursor was achieved in 73% yield, measured after purification by HPLC. [125I]MRS5127 bound to the human A3AR expressed in membranes of stably transfected HEK 293 cells. Specific binding was saturable, competitive, and followed a one-site binding model, with a Kd value of 5.74 ± 0.97 nM. At a concentration equivalent to its Kd, non-specific binding comprised 27±2% of total binding. In kinetic studies, [125I]MRS5127 rapidly associated with the hA3AR (t1/2 = 0.514 ± 0.014 min), and the affinity calculated from association and dissociation rate constants was 3.50 ± 1.46 nM. The pharmacological profile of ligands in competition experiments with [125I]MRS5127 was consistent with the known structure-activity-relationship profile of the hA3AR. [125I]MRS5127 bound with similar high affinity (Kd, nM) to recombinant A3ARs from mouse (4.90 ± 0.77), rabbit (2.53 ± 0.11), and dog (3.35 ± 0.54). For all of the species tested, MRS5127 exhibited A3AR agonist activity based on negative coupling to cAMP production. Thus, [125I]MRS5127 represents a new species-independent agonist radioligand for the A3AR. The major advantage of [125I]MRS5127 compared with previously used A3AR radioligands is its high affinity, low degree of non-specific binding, and improved A3AR selectivity.