A radio-immunoassay method of parathyroid hormone in peripheral circulation is described utilizing both commercially available antibody and bovine PTH preparation for iodination. As a standard, the bovine PTH delivered from the Medical Research Council (MRC) is used. Antibody-bound hormone is separated from free hormone by solid phase immunoprecipitation. Good cross reactivity of this antiserum to human PTH is shown, but because of the lack of human standard, it is demonstrated, that results may be expressed in reference to the MRC bovine hormone. The coefficient of interassay variation is 12% and the detection limit is 300 pg/ml. In every healthy subject, immunoreactive PTH is detected: the normal distribution range (n equals 31) was 400 to 1800 pg/ml. Two hypoparathyroid patients have undetectable levels. There is a slight overlap between normal and those from hyperparathyroid patients. The present method gives results expressed in international units (MRC standard). The reliability of the PTH radio-immunoassay is dependent on the characteristics of the antiserum. This method provides a basis for the establishment of PTH values with the possibility of comparing results in health and disease.