Both the parathyroid hormone (PTH) and the functionally similar parathyroid hormone-related protein (PTHrP) have served as templates for the development of novel bone anabolic agents for the treatment of osteoporosis. The PTHrP analog RS-66271 (Vickery, B. H.; Avnur, Z.; Cheng Y.; Chiou, S.-S.; Leaffer, D.; Caulfield, J. P.; Kimmel, D. B.; Ho, T.; Krstenansky, J. L. J. Bone Miner. Res. 1996, 11, 1943-1951), in which the amino acids 22-31 have been substituted by the sequence E22-L-L-E-K-L-L-E-K-L31 (a model amphiphilic peptide), is a potent bone anabolic agent in vivo. Therefore, RS-66271 is a good candidate for structural analysis with the aim of developing a structure-activity relationship. The structural characterization described here was carried out in aqueous solution employing circular dichroism and nuclear magnetic resonance spectroscopy. We find that the incorporated amphiphilic decapeptide is indeed helical. In addition, it induces the adjacent residues, up to residue 16, to adopt the helical conformation. The helical domain, including residues 16-32, incorporates most of the previously identified principal receptor binding domain PTHrP(25-34). We discuss the relevance of the distinct and extensive helicity in light of the reduced in vitro receptor affinity/ activity and the enhanced in vivo bone anabolic efficacy of RS-66271.