A highly purified and potent gonadotrophin in pregnant mare serum has been prepared. The preparation has been shown to be electrophoretically homogeneous in the Tiselius apparatus. The mobilities of the substance have been determined over a wide range of hydrogen ion concentrations.

The isoelectric point lies at pH 2.60–2.65 and the value of

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is 4.0 x 10–5.

Some chemical constituents have been studied. From the tryptophane and tyrosine content the molecular weight of the hormone is estimated to be 30,000.

The hormone has been subjected to acetylation by ketene in aqueous solution at room temperature and the result suggests again the essentiality of free amino groups for the biological activity of the hormone. In this respect it is to be contrasted with human chorionic gonadotrophin.

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