The results of cross-agglutination and agglutinin absorption experiments with the motile smooth, non-motile smooth, motile rough, and non-motile rough forms of B. typhosus are presented. Cross-agglutination between these four forms is complete, save that the motile rough antigen is under certain conditions only weakly agglutinated by the antisera prepared with the non-motile forms. Cross-absorption of the somatic agglutinogen of the four variants is complete, save that the motile smooth culture still shows granular agglutination with the anti-MS and anti-MR sera after absorption with these cultures. A theory of the antigenic composition of the four variants of B. typhosus is presented, based on the results obtained in these experiments. It would appear that, contrary to the usually accepted theory, the four variants have a common somatic agglutinogen. To explain the difference between the smooth virulent forms and the rough non-virulent forms it has been assumed that the S forms contain a carbohydrate which is associated with virulence and which takes no part in the agglutination reaction.