The antigenic relationship of S. alkalescens types I, II, III and IV to S. paradysenteriae was investigated. The following results were obtained: 1. (1) S. alkalescens types I and II are not agglutinated by mono-type-specific anti- S. paradysenteriae sera active against the fourteen types included in the Flexner dysentery bacillus group. Therefore, they do not share the type-specific antigens of S. paradysenteriae .2. (2) S. alkalescens type III, represented by strain #2-193, is strongly agglutinated (slide-agglutination test) by the mono-type-specific anti-Flexner type XIII (Boyd's type P143) serum. The other anti- S. paradysenteriae types I to XII and type XIV sera do not react with S. alkalescens type III. Quantitative agglutination and agglutinin-absorption tests indicate that the major type-specific antigens of S. alkalescens type III and S. paradysenteriae type XIII are different and that both organisms share a minor antigenic fraction.3. (3) S. alkalescens type IV, represented by strain #9731, is not agglutinated by any mono-type-specific anti- S. paradysenteriae sera with the possible exception of anti-Flexner type XIII serum. This latter serum weakly agglutinates S. alkalescens type IV. The major type-specific antigens of these two microorganisms are different.4. (4) Three types of S. alkalescens differ antigenically sufficiently from each other as well as from S. paradysenteriae types I to XIV to warrant recognition as distinct types of S. alkalescens . The taxonomic position of type II of Assis remains to be determined. In case this type be removed from the species, S. alkalescens types III and IV should be reclassified as S. alkalescens types II and III, respectively.
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