Hypercalcaemia always results in serious clinical sequalae and, if not treated, carries a most unfavourable prognosis. The clinician will gain major diagnostic help from an evaluation of the calcitonin and parathyroid hormone blood levels. With regard to parathyroid hormone we have developed, for the first time, a radioimmunoassay which is specific for the estimation of biologically active hormone in the circulation. We are dealing here with an unusual radioimmunological situation as the immunochemical sites are generally quite distinct from those associated with hormonal activity. We are presenting in this first paper the normal values and also the variations that occur in different types of hypercalcaemia. The comparison of these results with those obtained by the usual methods of estimation for parathyroid hormone assay lacking in biological activity shows the value of this new technique.