1. In this study a series of experiments showing the antigenic relationship of type 10 and type 12 group A hemolytic streptococci is reported. Agglutinin, precipitin, and passive protection tests with unabsorbed and reciprocally absorbed antisera were employed to show that representative type 10 and 12 strains contain serologically identical M antigens but unrelated type-specific T antigens.
2. Twenty-three other strains of group A hemolytic streptococci previously classified as either type 10 or type 12, collected from various sources, were shown to have an antigenic composition similar to one or the other of the two representative type strains.
3. The relationship of the two specific antigens, M and T, must be considered when any method employing the agglutinin reaction for the typing of group A hemolytic streptococci is used.