Parathyroid hormone (PTH) plays a critical role in mineral ion homeostasis, particularly in bone and kidney (Mannstadt et al., 1999; Gensure et al., 2005). A related peptide, parathyroid hormone-related peptide (PTHrP), plays an important role in skeletal development. Both peptides exert their biological actions by binding to a class B GPCR, PTH1, that signals primarily by activation of adenylate cyclase (Schipani et al., 1993). Mutations in PTH1 have been identified as the cause of Jansen's metaphyseal chondrodysplasia, Blomstrand's chondrodysplasia, and enchondromatosis (Schipani and Provot, 2003). Intermittent treatment with PTH has important clinical utility in building bone mass in patients with osteoporosis (Rosen, 2003). The cloned human PTH1-expressing cell line is made in the Chem-1 host, which supports high levels of recombinant PTH1 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 antagonists of interactions between PTH1 and its ligands.