1. A simple and accurate method of measuring the antitryptic indices of blood-sera is described.

  2. It is shown that for varying proportions of antitrypsin (= A) added to a specified amount of trypsin (regarded as unity) the relation: holds good for any given serum, T being the proportion of the trypsin neutralized by the serum and C a constant which is a direct measure of the number of molecules of antitrypsin contained in a specified volume of the serum.

  3. The molecular concentration of antitrypsin in blood serum is in great excess of the molecular concentration of proteolytically active material in a 1 per cent solution of Gruebler's trypsin.

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