Theoretically and practically syphilis is curable. However, it is impossible to decide when the last spirochete has been eradicated in the individual patient. This question is nevertheless of grave moment to the community, to the infected person and to the physician. Under modern conditions clinical and serologic "cure" is readily obtained in most instances. Radical biologic cure with complete destruction of the last spirochete is a therapeutic ideal which probably is attained frequently when intensive and continuous therapy is used during the early stages of the disease, especially in patients with primary syphilis, whose serologic reactions are negative. At present there is no reliable test for the determination of the presence or absence of the spirochetes in the human body. The various serologic tests merely indicate a reaction on the part of the body against the spirochete. When these tests are negative there is no proof that the spirochete