Parathyroid hormone has been isolated from hyperplastic parathyroid glands of vitamin D—defident chicks using standard methods including fractional precipitation, gel nitration, and ion exchange chromatography. Based upon SDS polyacrylamide gel electrophoresis, and elution positions from Sephadex and carboxymethylcellulose columns, the chicken hormone is similar in size and charge to its bovine counterpart. In a mouss calvarium assay system, the partially purified hormone stimulated calcium release, and inhibited citrate metabolism with a potency of 800 U psr mg. It cross—reacted weakly with three antisera to bovine parathyroid hormone. When chick glands were incubated with 3H—leucine and subsequently processed through the ion exchange chromatographic step, a radioactive fraction was detected which eluted from carboxymethylcellulose later than 3H—PTH, at a conductivity similar to that of the recently discovered peptide precursor of bovine parathyroid hormone named Calcemic Fraction—A, or proparathyroid hormone. This fraction eluted earlier than bovine parathyroid hormone upon gel filtration and reacted with two of the three anti— PTH antisera with which it was tested. These results suggest that it contained chicken proparathyroid hormone. (Endocrinology92: 1312, 1973)