The immunological properties of a synthetic peptide comprising the carboxyl-terminal 53–84 region of human parathyroid hormone (PTH) have been studied. The immunoreactivity of the synthetic human PTH-(53–84) peptide paralleled that of a 53–84 fragment of the native human hormone prepared by enzymic digestion, in both a standard radioimmunoassay, which was not region-specific, and also a radioimmunoassay specific for the carboxyl-terminal region of PTH. However, in both types of radioimmunoassay the synthetic human PTH-(53–84) peptide was four to five times more reactive than the native human PTH-(53–84) fragment.