C. equi possesses species specific antigens which are demonstrable by complement fixation tests. These antigens were not apparent in other species of diptheroids used for comparison. Type-specific antigens may be demonstrated by agglutination tests, or by precipitation tests in which acid extracts of the bacilli serve as antigens. Of the 20 cultures studied, 14 belonged to 2 main serologic types. Cultures from the genital tract of mares, from aborted equine fetuses and from pneumonia of foals belonged to the same serologic type. Cultures from hogs and buffaloes were demonstrated to be members of the species, indicating that C. equi may attack a variety of domestic animals.