This article is only available in the PDF format. Download the PDF to view the article, as well as its associated figures and tables. In the course of our work on blood clotting, we came on a reaction which seems to differentiate typhoid fever from other infections. The reaction is given early in the disease, being therefore much more useful in diagnosis than is the Widal test. The difficulty of obtaining a positive blood culture, together with the delay in the development of a positive Widal reaction, gives added value to a reaction which is reliable and present throughout the active period of the disease. This reaction also sharply differentiates typhoid fever from another disease with which it is often confused, miliary tuberculosis. We were engaged in measuring the antithrombic activity of the blood serum under a variety of conditions when we found that typhoid fever was differentiated from other fevers studied by us by a most intense antithrombin production throughout the fever period. An excess antithrombin production was found in cases of pulmonary