1. In the estimation of freely reacting SH-group of milk proteins by the electric photometer, the Kajita method is modified so as to use ammonium sulfate, (NH3)2SO4, in deproteinization-defatting processes. 2. The quantity of the freely reacting SH-group of milk proteins is greatest in regulated powdered milk dissolved to the concentration of the whole milk, followed by that in fresh human milk and milk on the market. 3. Even when the arsenic acid solution is made to act on liquid milk, no decrease in the freely reacting SH-group of milk protein can be recognized. In other words, no binding occurs between the freely reacting SH-group arsenic.