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The blood serum of the pregnant mare has been found to contain large amounts of an ovary-stimulating substance (Cole and Hart, 1). We have some evidence that the urine of pregnant mares is relatively free of this factor.* It has been demonstrated that in the human the ovary-stimulating substance is more concentrated in the urine than in the blood serum. These facts suggested the possibility that the substances from the two sources, producing an apparently similar biological reaction in the rat, might be different in chemical or physical composition. Evidence has been obtained by Collip (2) which indicates that the ovary-stimulating material prepared from placenta may not be identical with the hormone in the urine of pregnant women. In the present investigation, an attempt has been made to determine some of the chemical properties of the ovary-stimulating factor present in mare serum during pregnancy. In this paper we have designated as the ovary-stimulating principle, from whatever its source, that factor...

ALTHOUGH EVANS ET AL. (i) and Hellbaum (2) reported the separation of mare serum into follicle stimulating (FSH) and luteiriiziing (LH) fractions, critical confirmatory studies are still lacking. Our inability to confirm these findings (3) led us to adopt a new approach to the problem; that is, instead of attempting to fractionate two types of gonadotropic hormones we endeavored to fractionate gonadotropic hormone from the inert material in mare serum. Using the method of purification proposed by Cartland and Nelson (4) together with an isoelectric precipitation procedure of our own, we succeeded in preparing extracts testing 4000 to 7000 R.U. per mg. total solids (5). In the original serum there were approxi′mately 2 to 4 R.U. per mg. total solids (the serum assayed 200 to 400 R.U. per cc.) as compared to 7000 R.U. per mg. in the best purified material.

AMONG THE METHODS used for separating the gonadotropic hormone from pregnant mare serum, the fractional precipitation method of Cartland and Nelson (1) has proved most suitable, in our hands, for consistently yielding fractions of high potency with low nitrogen and solids content. We are now reporting some experiments upon the purification of the hormone obtained from very potent sera from pregnant ponies, testing 100 to 400 rat units1 per cc, together with some studies in which the starting material was obtained from less potent mare sera. The blood was collected from mares pregnant 65 to 75 days. The freshly drawn blood was defibrinated by whipping, and the serum obtained by centrifuging out the cellular constituents. Each batch of serum was worked up immediately; that is, treated according to the Cartland-Nelson process through the first precipitation of hormone by 70% acetone with the exception of two modifications.