1. The enzyme which decomposes the capsular polysaccharide of Type III Pneumococcus is associated with a protein which under optimal conditions of filtration passes through membranes with an average pore size of 10.6 mmicro but is held back by pores having a diameter of 8.2 mmicro. 2. When enzyme solutions are filtered to dryness through membranes of such porosity as to hold back the active principle, and when proper precautions are taken to prevent or minimize adsorption, the enzyme can be completely recovered in solution by immersing the membrane in distilled water or physiological salt solution. 3. These results are discussed with reference to the dimensions of the enzyme particle, and to the purification obtained in the course of ultrafiltration. 4. A practical method is described for the concentration and purification of the crude enzyme preparation by the use of graded collodion membranes.