The phenomenal growth in our knowledge of parathyroid hormone (PTH) physiology, chemistry and radioimmunoassay in the past 15 years has produced a significant increase in the use of the assay in the clinical laboratory evaluation of patients with disorders of calcium homeostasis. Recent experience with assays that have specificities for different regions of the amino acid sequence of the hormone and that can thus measure different portions of the total immunoreactivity in blood suggests that there may be different clinical applications for such assays. This report describes two different radioimmunoassay procedures and the clinical experience with each and suggests how each assay may be utilized in clinical evaluation of possible parathyroid dysfunction. The assay for carboxyl-terminal PTH 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 tissues, 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.