Two different radioimmunoassay procedures and the clinical experience with each are described, and how each assay may be utilized in clinical evaluation of possible parathyroid dysfunction is suggested. The assay for carboxyl-terminal PTH [parathyroid hormone] is more useful in the differential diagnosis of the possible causes of hypercalcemia; the intact PTH assay is preferred in selective venous catheterization for preoperative localization of hyperfunctioning tissue, and both assays have usefulness in the evaluation of patients with hypocalcemia. In chronic renal failure, the considerations are more complex. In many patients, the intact PTH assay is preferred for monitoring the clinical course; however, in other patients the carboxyl-terminal PTH assay has been more useful. The best assay for each patient must be determined by initial evaluation with both assays.