Virulence and Immunogenic Activities of<i>B. Typhosus</i>in Relation to its Antigenic Constituents

1. Rough variants, derived from strains of B. typhosus which possess the Vi antigen, may still contain this antigen though the smooth O antigen has been lost. 2. Such variants, which are non-virulent, are yet capable of inducing active and passive immunity. 3. The virulence of B. typhosus depends on the combined activity of the smooth O and the Vi antigen. Nevertheless the Vi antibody alone is sufficient to protect against infection with strains of the highly virulent type. 4. The use of avirulent, rough, but Vi-containing variants as vaccines and in the preparation of therapeutic sera is discussed.