Recently, the highly selective mu, delta and kappa radiolabeled opioid ligands, such as 3H-DAGO (mu ligand), 3H-DPDPE (delta ligand) and 3H-U69593 (kappa ligand) are available. Using the kappa-homogeneous preparations from human placenta and mu-enriched gerbil cerebellum membrane preparations with these highly selective radiolabeled opioid ligands, clinically used opioids were tested for their mu, delta and kappa properties. The mu agonists such as morphine and fentanyl display a very low affinity for delta and kappa receptor. Among the agonist-antagonist, buprenorphine and butorphanol appeared to be the most highly selective agonists for mu and kappa opioid receptors, respectively. Most ligands identified as specific agonists are in fact only selective and appear to interact at more than one receptor type.