Extracellular adenosine mediates a multitude of biological effects, including wakefulness, antiarrythmia, bronchoconstriction and response to ischemia and oxidative stress. A family of four G protein-coupled adenosine receptors, A1, A2A, A2B and A3, is responsible for these effects. A2B, which couples to Gs, is expressed in smooth muscle of the bowel, and blood vessels, and mediates relaxation of these tissues in response to adenosine (Fredholm et al., 2001). Airway mast cells also express A2B, which appears to contribute to bronchoconstriction during asthma (Holgate, 2005). Chemicon’s cloned human A2B -expressing cell line is made in the Chem-3 host, which supports high levels of recombinant A2B expression on the cell surface and contains high levels of the promiscuous G protein Gα15 to couple the receptor to the calcium signaling pathway. Thus, the cell line is an ideal tool for screening for agonists and antagonists at A2B.