2. The protein nature of the M antigen, present in matt variants only, has been demonstrated in previous work, but the chemical characteristics of the newly recognized antigenic factor, T, present in both variants, have not been determined. This T factor is identified only by its immunological reactions. It is unknown whether the two type-specific antigenic factors, M and T, occur as separate chemical entities in the matt variant or in conjugation.

3. Antibody to the type-specific protein, M, appears responsible for the M precipitin reaction, for type-specific protection, and, as a rule, for part of the type-specific agglutination of matt variants, but in type 1 it does not cause agglutination.

4. Antibody to the second type-specific antigen, T, seems to be solely responsible for type-specific agglutination of the glossy form and to play a large rôle in type-specific agglutination of the matt form, but apparently it is not involved in protection. This T antibody causes all of the type-specific agglutination of type 1. Consequently, type 1 matt and glossy variants agglutinate and absorb agglutinin alike, and antisera to both are identical in content of type-specific agglutinin though they differ in respect to M antibody.

5. Recognition of the principle underlying type-specific agglutination of glossy variants makes it possible to suggest, with certain reservations, the use of glossy variants for type classification by agglutination. These variants are suitable for preparing type-specific agglutinating antisera, and they form stable suspensions for use in the reaction. Improved methods are needed for deriving glossy from matt variants.