In drawing the polarographic curves of catalytic wave of protein and reductive wave of arsenic after adding arsenic acid to various milk solutions, by variation in the concentration of arsenic acid or proteins, changes occurred in the shape of these waves due to the action of surface activity of milk proteins. On comparing the action of surface activity of human milk protein with that of cow's milk protein, the following results were obtained. 1. As for human milk and cow's milk, when the concentration of cow's milk protein is adjusted so as to contain the same concentration of protein in human milk, it has been found that human milk protein possesses a superior action of surface activity than that of cow's milk protein. 2. In comparing the action of surface activity of human milk protein and cow's milk protein after changing the quality of milk protein by heating, it has been found that the surface activity of human milk pretein is greatly decreased by boiling, while that of cow's milk protein is decreased only slightly. 3. In comparing the action of surface activity of various milks by adjusting the pH of hydrochloric arsenic acid solution to 4.5 which is close to the pH of gastric juice of infant and to 2.5 which is the pH of gastric juice of adult, and when these solutions are made to act on milk, the most marked action of surface activity can be observed in human milk at the pH of gastric juice of infant. while in cow's milk at pH of gastric juice of adult. Moreover, at the pH of gastric juice of infant, the action of surface activity is superior in raw milk than heated milk in the case of human milk, while in heated milk rather in raw milk in the case of cow's milk.