Soluble specific substances were prepared from 3 representative strains of Group C (formerly Group II α) meningococci. Dilutions as high as 1:4,000,000 of all preparations gave positive precipitin reactions with homologous rabbit antiserum. However, all evidenced some degradation of immunologic integrity, e.g., the best preparation absorbed only ¾ of the homologous mouse-protective activity from a rabbit Group C antiserum and it failed to engender significant active immunity in mice. On the other hand, intraabdominal injection of 30 μg of this preparation/animal sharply diminished the immune response of mice to subsequent injections of an effective Group C vaccine. The precipitin reactions of these substances with an equine polyvalent antimeningococcal serum prepared in 1918 indicated that Group C meningococci had been isolated in the United States at that time, although they were not recognized as a distinct group until 1940.