Abstract
1. The Donnan equilibrium furnishes a test for the ionic nature of any diffusible substance, since the ratio of the concentration of any ion on the two sides of a membrane must be equal to the ratio of the concentrations of any other ion of the same sign and valence, whereas a non-ionic substance would be equally distributed on both sides. 2. The distribution of trypsin inside and outside of gelatin particles has been compared to the distribution of hydrogen and chloride ions under the same conditions. 3. The ratio of the trypsin concentration in the gelatin to the concentration in the outside liquid is equal to the ratio of the hydrogen ion under the same conditions and to the reciprocal of the chloride ion ratio. 4. This result was obtained between pH 2.0 and 10.2. At pH 10.2 the trypsin is equally distributed and on the akaline side of 10.2 the ratio is directly equal to the chloride ratio. 5. Trypsin is therefore a positive monovalent ion in solutions of pH 10 to 2. It is probably isoelectric at 10.2 and a monovalent negative ion on the alkaline side of 10.2 6. Trypsin must also be a strong base since there is no evidence of any undissociated form on the acid side of pH 10.2.