[en] Radioimmunoassay -- rather than immunoradiometry, bioassay or cytochemical assay -- is the PTH determination of the future. The commercially available radioimmunoassays have been developed with nonhuman reagents. Those using antisera that react with C-terminal circulating fragments distinguish normal from hyperparathyroid subjects better than those that recognize the intact PTH. The i-PTH has utility in selective venous catheterization for preoperative localization of hyperfunctioning parathyroid tissue. However, the ability of these antisera to detect human N- and C-terminal fragments has been determined by using bovine PTH fragments, since fragments of human PTH are not commercially available. The production of antibodies by humans or animals is dependent on the injected antigen causing the host to make antibodies. Antibody may not only differ from species to species but from individual to individual within the same species. Antibody differences can also be encountered from one immunization to another. These factors account for the nonspecificity and noncorrelation differences between assays, and do not reflect the true in vivo situation