FROM the point of view of treatment the history of syphilis may be divided into two periods, the first prior to and the second subsequent to March 3, 1905, the date of Schaudinn's (1) discovery of the Spirochaeta pallida. The division is justifiable since the accurate diagnosis and efficient treatment of the present day owes its being entirely to Schaudinn's discovery. An inevitable corollary to the discovery of a specific microorganism is a research into the antibodies to it which are produced by the infected host, and a by-product of such a research in the case of syphilis was the discovery of the Wassermann-Neisser-Bruck reaction (2), to give it its full original name. The argument as to whether the Wassermann reaction is a true Bordet-Gengou phenomenon (3) is beside the purpose of this review. It is sufficient from the point of view of treatment that it complemented the discovery of the Spirochaeta pallida in laying the foundation for exact methods of research into the value of antisyphilitic remedies. The tripod on which the modern therapy of syphilis mainly rests was completed by Ehrlich's discovery (1909) of dioxy-diamido-arsenobenzol dihydrochloride, known first as '606', or the 'Ehrlich-Hata remedy', and later as 'salvarsan '. In acknowledging the debt which the modern therapy of syphilis owes to these three discoveries it would be unjust not to mention that Bordet and Gengou (4) had probably seen the Sp. pallida in 1903, and that it was their work on complement fixation which laid the foundation for the Wassermann test. It is also very largely due to the work of Metchnikoff (4) and Roux on the transmission of syphilis to monkeys that the Sp. pallida was accepted so quickly. THE REMEDIES USED IN THE TREATMENT OF SYPHILIS.