Humoral (H) endorphin, a novel endogenous factor with opiate-like activity was further characterized using various radioreceptor assays. H-Endorphin displaced both labelled opiates (morphine, ethylketazocine) and opioid peptides (leucine-enkephalin) from their specific binding sites in rat brain membranes. The effect of sodium ions on H-endorphin indicates its agonistic nature which is in agreement with previous pharmacological experiments. However, H-endorphin potentiated the binding of the opiate antagonist naloxone to brain membranes. This peculiar effect of H-endorphin on naloxone, together with the non-conventional interactions observed in various pharmacological assays, clearly distinguishes H-endorphin from other opioid ligands. A multisite model of the opiate receptor is presented. This model, which is compatible with the well documented evidence for receptors heterogeneity also explains non-conventional interactions between various opioid ligands.