The components of secondary structure of the biologically-active N-terminal domain of human parathyroid-hormone-related protein (residues 1-34) and several truncated species were examined using Fourier transform infrared (FTIR) spectroscopy. The major structural features include a segment of alpha-helix within the N-terminal segment probably extending from Glu-4 to Lys-11 with three beta-turns localized to the segments Gly-12 to Ile-15, Gln-16 to Arg-20 and His-25 to Ala-29. Some beta-sheet was detected in the full-length peptide, but not in any of the C-terminal truncated samples. These structural features were studied in the smaller peptides for the purpose of localization of the various components and with a view to describing the region likely to form the bulk of the receptor binding site.