1. Sodium soaps prepared from olive oil, croton oil, cod-liver oil, linseed oil, etc., have the property of inhibiting the action of trypsin and leucoprotease. 2. The activity of these soaps is dependent upon the degree of unsaturation of the fatty acids and is in proportion to their iodin value. 3. Saturation of the acids with a halogen (iodin) causes a loss of this property. 4. Soaps of the saturated fatty acids tested do not have this influence on ferments.