1. In observing the polarographic catalytic wave of protein by making arsenic acid at various concentrations act on fresh human milk, fresh cow's milk, regulated powdered milk, and milk on the market, no decrease in the degree of the SH activity of milk protein due to the addition of arsenic can be detected. In this instance when the concentration of arsenic acid is gradually increased to a certain point, the reductive wave of arsenic appears on the heel of the proteinic wave. The arsenic acid concentration required to have the appearance of such an arsenic wave is 6 γ/cc in the case of fresh human milk; 5 γ/cc in fresh cow's milk; 4 γ/cc in regulated powdered milk; and 3 γ/cc in milk sold on the market. 2. In drawing the polarographic curve of the reductive wave of arsenic by addition of milk at various concentrations to a hydrochloric arsenic acid solution, the arsenic wave falls in proportion to the addition of milk protein. The percentage of the fall in the amplitude of arsenic wave is greatest in fresh cow's milk followed by milk on the market regulated powdered milk and fresh human milk in the order mentioned. 3. The rate of decrease in the amplitude of the arsenic wave in fresh human milk and the gelatin-protein of an equal concentration is greater in the latter. Therefore, it is obvious that the reaction between milk protein and arsenic is in no way associated with the SH-group. 4. By applying the Langmuir's formula to these results, it has been confirmed that the reactions are induced by the action of surface activity of the milk proteins.