1. A phage-resistant variant of Phase II Shigella sonnei has been isolated and the type-specific antigen from it has been obtained.
2. The antigen of the variant microorganism, II/3, 4, 7, differs from that of the parent Phase II bacillus in serological specificity and in chemical make-up.
3. Although the antigen of Phase II Sh. sonnei inhibits all T phages to which the bacillus is susceptible, that of the variant fails to inhibit T3, T4, and T7.