The first therapeutic studies of arsphenamine were carried out by Ehrlich and Hata (1) on mice and rats infected with the organism of relapsing fever, on fowls with fowl spirillosis, and on rabbits with experimental syphilis. Rabbits with scrotal chancres were treated with the drug and the rate of healing of the lesion and of the disappearance of treponemata was noted. Rapid disappearance of the clinical phenomena and of the treponemata was interpreted by Ehrlich and Hata to mean that the animals had been sterilized, in other words, that complete biological cure had been obtained. The more recent knowledge of the behavior of syphilis in the rabbit permits us to say that these criteria of cure are altogether insufficient.Kuznitsky (2), working under Neisser's direction, studied the effects of arsphenamine upon syphilis in apes and obtained what he regarded as certain cure in six out of nine animals treated. The criterion for cure adopted by this worker was the success or failure of reinoculation. If reinoculation proved to be successful the animal was regarded as having been rid of all its spirochetes; if unsuccessful, it was thought that infection still existed. In therapeutic experiments with other arsenicals and with various mercurials Neisser (3) had used either the reinoculation method as a criterion of cure, or had inoculated emulsions of internal organs (liver, spleen, bone marrow) of treated animals into normal apes to determine whether