Abstract
Phase I and Phase II Sh. sonnei exhibit differences in their susceptibility to the T series of bacteriophages. Both microorganisms are lysed by T(2) and T(6), but only the Phase II bacillus is lysed by T(3), T(4), and T(7). Lysis of the Phase I or Phase II bacillus by T(2) or T(6) is not inhibited by the homologous type-specific antigen. In the presence of an appropriate cofactor, however, the lysis of Phase II Sh. sonnei by T(3), T(4), and T(7) is specifically inhibited by the homologous somatic antigen but not by the Phase I antigen. The significance of these observations is discussed in respect to the nature of the virus receptor of these microorganisms.