The beta adrenergic receptors mediate the effects of endogenous catecholamines, such as epinephrine, by coupling to Gs to stimulate cAMP . Whereas β1 and β2 are found predominantly in heart, the β3 receptor is found primarily in adipose tissue. Activation of adipose β3 results in lipolysis and thermogenesis. A polymorphism in the human gene for β3 is associated with weight gain in obese patients (Clement et al., 1995). In addition, mice lacking the β3 -adrenoceptor display increased total body fat, particularly on a high fat diet (Revelli et al., 1997). These observations indicate that β3 is a possible target for obesity treatments. Millipore's cloned human β3 -expressing cell line is made in the Chem-10 host, which supports high levels of recombinant β3 expression on the cell surface and contains high levels of promiscuous G proteins to couple the receptor to the calcium signaling pathway. Thus, the cell line is an ideal tool for screening for antagonists of interactions between β3 and its ligands.