1. Type-specific antibodies were demonstrated by the indirect bacteriostatic test in sera from human adults convalescing from group A streptococcal infection of the upper respiratory tract. The time of appearance of the antibodies varied from 3 to 5 weeks; and they persisted in 2 patients for at least 37 weeks after the onset of the infection.
2. The specificity of the antibody response in one serum was tested with strains of 7 heterologous types; in another, with 6; and in the third, with 2; but in no instance were cross-reactions observed. Moreover, each convalescent serum showed approximately equal bacteriostasis for 7 different strains of the same type as that which caused the infection.
3. The antibodies were specifically absorbed from the serum by homologous heat-killed streptococci, but not significantly by strains of heterologous types.
4. The specific M antigen of the streptococcal cell with its respective antibody, and not the T substance, appeared to be concerned in the reaction.
5. In spite of numerous technical difficulties inherent in the method, this bacteriostatic test provides a useful procedure for studying type-specific immunity in streptococcal infections.