+The Recognition of Syphilis by the Blood Test.+--When the Wassermann test shows the presence of syphilis, we speak of it as "positive." Granted that the test is properly done, a strong positive reaction means syphilis, unless it is covered by the limited list of exceptions. After the first few weeks of the disease, and through the early secondary period, the blood test is positive in practically all cases. Its reliability is, therefore, greatest at this time. Before the infection has spread beyond the first sore, however, the Wassermann test is negative, and this fact makes it of little value in recognizing early primary lesions. In about 20 to 30 per cent of syphilitic individuals the test returns to negative after the active secondary stage is passed. This does not necessarily mean that the person is recovering. It is even possible to have the roof fall out of the mouth from gummatous changes and the Wassermann test yet be negative. It is equally possible, though unusual, for a negative Wassermann test to be coincident with contagious sores in the mouth or on the genitals. So it is apparent that as an infallible test for syphilis it is not an unqualified success. But infallibility is a rare thing in medicine, and must be replaced in most cases by skilful interpretation of a test based on a knowledge of the sources of error. We understand pretty clearly now that the Wassermann test is only one of the signs of syphilis and that it has quite well-understood limitations. It has revealed an immense amount of hidden syphilis, and in its proper field has had a value past all counting. Experience has shown, however, that it should be checked up by a medical examination to give it its greatest value. Just as all syphilis does not show a positive blood test, so a single negative test is not sufficient to establish the absence of syphilis without a medical examination. In a syphilitic, least of all, is a single negative Wassermann test proof that his syphilis has left him. In spite of these rather important exceptions, the Wassermann test, skilfully done and well interpreted, is one of the most valuable of modern medical discoveries.