Although the iodids have been employed in the treatment of syphilis for many years, their mode of action is still obscure. Clinical experience has shown that iodin compounds have little or no therapeutic effect in the early stages of the disease, due, it is asserted, to the absence of spirocheticidal action; and experimental confirmation of this opinion has been brought forward by Nichols,1 who found that the administration of the amount of potassium iodid tolerated by the body (rabbit) was without effect on the number and motility of the spirochetes of the primary chancre. On the other hand, Neisser2 reported that it was possible to prevent syphilis in the monkey with potassium iodid, if massive doses of from 7 to 8 gm. were administered subcutaneously at the time of or four days after inoculation; that is, no primary lesion was obtained; but Neisser does not state whether secondary