1. Type-specific anaphylactic shock was produced with HCI extracts of Streptococcus hæmolyticus in guinea pigs passively sensitized with antibacterial sera.
2. With occasional sera and certain HCl extracts, type-specific shock was not produced unless the serum was first absorbed with heterologous bacteria or unless the guinea pigs were desensitized with heterologous extracts before testing with the homologous extract. The findings indicated that this was due to the presence of the non-type-specific substance which has been provisionally designated as Y.
3. Tryptic digestion destroyed the ability of HCI extracts containing the type-specific M substance and the non-type-specific Y substance to produce anaphylactic shock in passively sensitized guinea pigs.
4. Active sensitization was not accomplished with the type-specific M. It seems probable, therefore, that this substance is a hapten, reacting with antibodies but not stimulating their production after separation from the bacterial cell.