Summary Precipitation of diphtheria-toxoid by alum depends on the pH under which the precipitate is formed. Purified toxoid requires much less amounts of alum to be precipitated than the original toxoid. One-eighth by volume of 4 per cent solution of alum seems to be sufficient for toxoid purified with calcium-phosphate to precipitate the greater part of the active principle from it. The superiority of the alum-toxoid as an immunizing agent was shown with toxoids purified with calcium-phosphate.