Acetone powder of rat kidney was extracted with0.15 m KCl containing 5 mM tris-Cl and 15% glycerol. The fraction, precipitated by 35-75% saturated ammonium sulfate, was chromatographed on a Biogel P-300 column to obtain a partially purified rat kidney enzyme preparation preferentially hydrolyzing parathyroid hormone. Parathyroid hormone in human serum lost its immunoreactivity after incubation with this rat kidney enzyme preparation according to radioimmunoassay using guinea pig antibody to bovine parathyroid hormone and <sup>125</sup>I-labeled highly purified bovine parathyroid hormone. Such activity of destroying PTH immunoreactivity, proportional to the amount of enzyme and duration of incubation, was lost after heating at 60°C for 10 min. The optimum pH, 8.6, and susceptibility to inhibitors such as p-chloromercuribenzoate, monoidoacetate, o-phenanthroline and EDTA were similar to those in the hydrolysis of <sup>125</sup>I-PTH.