Opiates derived from the opium poppy, Papaver somniferum, have been used in for millenia for their anti-diarrheal, analgesic and euphoric properties. More recently, endogenous peptides, enkephalins, dynorphins, and endorphins, were found to bind to the same sites as opiate alkaloids. The receptors for the classical opioids are three related GPCRs, µ, δ, and κ, that activate Gi/o to reduce intracellular cAMP levels. Most clinically used opioids function by activation of the µ opioid receptor (Dhawan et al., 1996). Chemicon's cloned human µ-expressing cell line is made in the Chem-5 host, which supports high levels of recombinant µ expression on the cell surface and contains high levels of the promiscuous G protein to enhance coupling of the receptor to the calcium signaling pathway. Thus, the cell line is an ideal tool for screening for antagonists of interactions between µ and its ligands.