1. A rapid and simple method is given for the approximate determination of the specifically precipitable protein in Type I antipneumococcus sera. 2. It is shown that a close parallel exists between the specifically precipitable protein and the number of mouse protection units in a wide variety of Type I antipneumococcus sera. 3. Owing to the consistent results obtained and the rapidity, simplicity, and economy of the method, its use is proposed instead of the mouse protection test as a basis for the titration of standard sera and the comparison of others with a standard. 4. A method is given for conveniently preparing highly purified specific polysaccharide of Type I pneumococcus.