With the recently solved crystal structure of the murine mu opioid receptor, the elucidation of the structure function relationships of the human mu receptor becomes feasible. In this study, we analyzed the available structural information along with ligand binding and G protein activation of human mu receptor. Affinity determinations were performed in a HEK293 cell line stably transfected with the human mu opioid receptor for 6 different agonists (morphine, DMAGO, and herkinorn) and antagonists (naloxone, beta-Funaltrexamine, and Norbinaltorphimine). G protein activation was investigated in membrane preparations containing human mu receptors treated with the agonist, partial agonist, or antagonist compounds. 4DKL.pdb was utilized for structural analysis and docking calculations for 28 mu receptor ligands. The predicted affinities from docking were compared with those experimentally determined. While all known ligands bind to the receptor through the same binding site that is large enough to accommodate molecules of various sizes, interaction with D147 (D149 in human mu receptor) is essential for binding. No distinguishable interaction pattern in the binding site for agonist, partial agonist, or antagonist to predict pharmacological activities was found. The failure to reconcile the predicted affinities from docking with experimental values indicates that the receptor might undergo significant conformational changes from one state to the other states upon different ligand binding. A simplified model to understand the complicated system is proposed and further study on these multiple conformations using high resolution structural approaches is suggested.