1. The occurrence of closely related T antigens in the series composed of types 15, 17, 19, 23, and 30 accounts for most of the cross reactions observed among these types. Similarly T antigens, unrelated to the first series but mutually related, occur in a second series comprising types 4, 24, 26, 28, 29, and 46.
2. Matt variants of each of the eleven types studied possess type-specific M antigens demonstrable either by precipitin or agglutinin reactions.
3. In seven of these types, strains have been encountered which do not possess the T antigen usually associated with the type in question.
4. Procedures are outlined in the appendix for preparing specific antisera for the classification of these types by the slide agglutination technique.