The method used in the extraction of the parathyroid hormone is a vigorous treatment involving boiling in a solution of a strong mineral acid in order to liberate the activity, possibly as a result of some hydrolytic action.1 The methods that yield the active principles from other endocrine glands do not yield the activity of the parathyroid, whereas the method that has proved successful for the parathyroid is known to decompose most of the other known active endocrine principles. Two methods are available for the standardization of active parathyroid extracts on dogs. The method proposed by Collip2 is based on the production of hypercalcemia, an abnormal condition, in normal dogs. The method that I find most satisfactory is based on my early work on dogs in which operation had been performed.3 Following the operation of parathyroidectomy there is a rapid fall in the blood serum calcium, which