2. Mice may be actively immunized with the protein fraction obtained by treating pneumococci with anhydrous sodium sulfate or by solution of pneumococci in bile salts and precipitation with alcohol. Pneumococcus antigen is therefore carried within or adheres to the protein fraction of the organism.

3. Pneumococcus antigen is resistant to prolonged autolysis and to tryptic digestion, and can be recovered from the soluble portions of digests of either the intact bacteria or the bacterial protein.

4. The antigen may be isolated from each of the three fixed types of pneumococcus by tryptic digestion of the pneumococci and extraction of the digest with 70 to 90 per cent alcohol. The antigen is not soluble in absolute alcohol, nor is it soluble in ether or in the other lipoidal solvents.

5. The immunizing property of slightly acid solutions of the antigen is not impaired by boiling for 5 minutes, nor by heating at 56°C. for 1 hour. Sterile unpreserved solutions of the antigen did not deteriorate by standing in the refrigerator for 3 months.

6. The exact chemical nature of pneumococcus antigen still remains to be determined. From the experiments reported it appears that the antigen is non-lipoidal, that it probably adheres to the protein fraction in a loose chemical or physical union rather than representing a protein complex of a large molecular size, as shown by its solubility in alcohol, its thermostability, and its resistance to proteolytic digestion.

7. Some of the antigens studied have been shown to contain a non-specific factor promoting the growth of bacteria.

8. The purified pneumococcus antigen solutions are non-toxic for mice.