1. At least three subgroups of Pneumococcus Type II may be recognized by specific immune reactions. They have been called Subgroups II A, II B, and II X.
2. That the organisms of these three subgroups are biologically related to Pneumococcus Type II is shown by the following facts:
(a) Agglutination with Antipneumococcus Serum II. (b) Protection with Antipneumococcus Serum II, except Subgroup II X. (c) Absorption of Antipneumococcus Serum II with typical Type II pneumococcus removes the antibodies for all subgroups, (d) Absorption of Antipneumococcus Serum II with a member of Subgroups II A or II B removes only the antibodies for the homologous subgroup. Absorption of Antipneumococcus Serum II with any given member of Subgroup II X removes the antibodies for that particular strain only.
3. That the three subgroups, although biologically related to Pneumococcus Type II, possess, nevertheless, specific differential characterswhich separate them one from another, is evidenced by thefollowing facts: (a) The organisms of any subgroup are not agglutinated bythe antisera of the other two subgroups. (b) They are not protected against by the sera of the other subgroups. (c) They do not absorb from Antipneumococcus Serum II the specific immune bodies of the other subgroups.
4. Subgroups II A and II B are characterized by immunity reactions identical within the respective group.
5. Subgroup II X consists of heterogeneous strains which do not cross in their immunity reaction with each other or with Subgroups II A or II B.