1. The examination of 1100 sera by both the Wassermann reaction and the complement-fixation test with spirochaetes revealed a superior sensitivity of the latter reaction and practically equal specificity of the two tests.
2. Syphilitic serum contains two different antibodies: one reacting with the lipoid antigen of the Wassermann reaction, the other with a specific antigen in the spirochaete.
3. The spirochaetal antibody of syphilitic serum has a complex serological structure, corresponding to spirochaete strains of different antigenic make-up.
4. The existence of this antibody and its specific absorption by the homologous antigen can also be demonstrated by agglutination.
5. The difference between agglutinin titres found in normal and syphilitic sera is not pronounced enough to render this method satisfactory for the practical diagnosis of syphilis.
6. The spirochaete contains, apart from its specific antigen, the ubiquitous lipoid substance representing the Wassermann antigen.
7. A fraction was obtained from spirochaetes by Raistrick and Topley's method which in complement-fixation and precipitation tests reacted actively with spirochaete antisera from rabbits, but which so far failed to react with syphilitic sera.
This work was carried out with the aid of a grant from the Rockefeller Foundation. I wish to thank Prof. Golla, Director of the Central Pathological Laboratory, L.C.C. Mental Hospitals, who rendered this work possible, and Dr Arthur Davies, Director of the Devonport Laboratory, for the hospitality afforded me at his laboratory and for the patients’ sera used in this work. I am indebted to Prof. R. T. Hewlett for his revision of the manuscript, to Prof. Raistrick for advice in chemical matters, and to Dr Amies, of the Lister Institute, for his help with the Sharples centrifuge.