Unexpected contamination of several jars of a culture-medium was traced to some red rubber stoppers which had been washed and boiled and then kept for some days in 1:1000 potassium mercuric iodide solution, which was exposed to aerial contamination. An examination of this solution showed that it had very greatly diminished antiseptic properties. Not only did a mixture of equal volumes of the solution and broth support the growth of Staphylococcus but living organisms were actually present in it. These observations were repeated with a second solution of pure mercuric iodide and potassium iodide and other samples of red rubber, and their confirmation led to the investigation of the action of several kinds of rubber upon potassium mercuric iodide solutions.