Paul Ehrlich and 606

To the Editor.  —Simon,¹ in his review of neurosyphilis for the Archives, limited his discussion of therapy to penicillin, but the editors (not including me) have cleverly added a reference to salvarsan, the organic arsenical that was the first agent to be used effectively throughout the world in the treatment of syphilis. Paul Ehrlich, its inventor, was both a biologist and a chemist; before 1910, when he formally announced the efficacy of salvarsen,² he had studied the specificity of antibodies—his "magic bullets"—and had shared the 1908 Nobel Prize for his research in immunity. Using similar concepts to improve the specificity of organic compounds that might kill parasites, he originated the field of chemotherapy.³Salvarsan was also known as 606, because it was the 606th compound to be tested by Ehrlich and his collaborators against such organisms as trypanosomes and the newly-identified spirochete. (Neosalvarsan was the 914th compound