Somatic soluble antigens of S. typhi can be subdivided into three categories. These are: 1. Thermolabile antigens (TL) at 100 degrees C or 121 degrees C. They showed two kind of fractions, by immunoelectrophoresis method, some anodic and others cathodic in migration. 2. Thermostable haptens (Th) at 100 degrees C or 121 degrees C. Demonstrated by their capacity to absorb antibodies after heat treatment (100 degrees C or 121 degrees C) and to precipitate antibodies from antiserum produced with unheated bacterial suspensions. These fractions failed to induce antibody production, after heating in rabbits. Electrophoretically they migrated toward the anode. 3. Thermostable antigens (TS) at 100 degrees C or 121 degrees C. Demonstrated by their capacity to induce antibody formation in rabbits after heat treatment at 100 degrees C or 121 degrees C and to precipitate antibodies after being heated at these temperatures. These antigens revealed only cathodic migration. Using the serum made with S. typhi strain Ty-21a, thermolabile and thermostable antigens were demonstrated in S. typhi 0901 strain.