(PTH) has permitted more definitive characterization of its chemical and immunological properties, as woll as its physiological behaviour1 _ 1n 1963, Berson, Yalow, Aurbach, and Potts2 reported the development of a specific and sensitive radioimmunoassay for the bovine hormono. In principle, this technique was similar to the mot. hods developed for tho measurement of insulin and growth hormone•, and depended on the inhibition by unlabelled hormono of tho binding of hormone labelled with iodine-131 to specific antibody. With recent improvements in the sensitivity of the assay, particularly the use of non-equilibrium conditions•, it has been possiblo to measure reproducibly tho concentration of PTH in normal bovine plasma and perform a number of physiological studies conoerning t, he regulation of hormone secretion•.