The recent cloning and expression of an opioid mu receptor has opened up new opportunities for research in opioid pharmacology. The relatively low level of transient receptor expression in COS cells emphasizes the need for radioligands with high specific activity and low nonspecific binding with which to label receptors. In addition, recent data indicating that agonists and antagonists bind to different domains on the same receptor protein indicate the utility of having both agonist and antagonist radioligands available for the study of opioid receptor mechanisms. Previous studies characterized the binding of the opioid antagonist 6 beta-[125iodo]-3,14-dihydroxy-17-cyclopropylmethyl-4,5 alpha-epoxymorphinan ([125I]IOXY) and showed that this naltrexone analog labels mu and kappa 2 receptors in rat and guinea pig brain with high affinity and low nonspecific binding. In the present study, we synthesized the agonist congener of IOXY, 6 beta-iodo-3,14-dihydroxy-17-methyl-4,5 alpha-epoxymorphinan. We named this novel agent IOXY-AGO for IOXY-agonist. Competition binding studies showed that IOXY-AGO has high affinity for mu receptors (Ki = 0.28 nM) and lower affinity for delta (Ki = 18.7 nM) and kappa 1 (Ki = 33.9 nM), kappa 2a (Ki = 38.4 nM) and kappa 2b (Ki = 58.2 nM) binding sites. IOXY-AGO was radioiodinated to a specific activity of 2,200 Ci/mmol. [125I]IOXY-AGO binding was rapid, readily reversible, and characterized by low nonspecific binding.(ABSTRACT TRUNCATED AT 250 WORDS)