1. Spirochetes of relapsing fever have been separated from the blood of heavily infected mice and rats by hemolysing with saponin, followed by repeated washing of the spirochetal suspension with physiological saline.
2. Spirochetes obtained in this manner appear to have broad antigenic specificity. Antigens of this type fixed complement in the presence of serum obtained from man or animals infected with one or other of the recognized strains or "species" of relapsing fever spirochetes. Macroscopic agglutination of the antigens likewise was observed with sera from the same sources.
3. Positive serological reactions were not observed with convalescent sera obtained following infection with other diseases, for example, typhus fever, malaria, Rocky Mountain spotted fever, Weil's disease, syphilis, and typhoid fever. Hyperimmune sera prepared against other pathogens also failed to react with the relapsing fever antigens.
4. No apparent change in the antigen occurred following storage in the ice box for as long as 4 months.
5. The results indicate that treatment of the spirochetes of relapsing fever with saponin yields a relatively stable antigenic preparation which may prove useful in the serological diagnosis of this disease.