Many infectious diseases have been reported as causing biologic false positive serologic reactions for syphilis. That some of them (malaria, leprosy and infectious mononucleosis) cause such reactions has been well established. Other diseases, e. g., relapsing fever, rat bite fever, scarlet fever, tuberculosis, pneumonia, Vincent's infections, malignant conditions (particularly cancer of the tongue), subacute bacterial endocarditis, glanders, Weil's disease, leishmaniasis, lymphogranuloma venereum, trypanosomiasis, typhus fever, vaccinia and infections of the upper respiratory tract, as well as injections of horse serum, have from time to time been suggested as causing such serologic reactions, but the evidence is as yet inconclusive. From a review of the literature it is impossible to arrive at any conclusions regarding the percentage of biologic false positive reactions obtained in any of these diseases except leprosy and infectious mononucleosis. Until recently it was thought that biologic false positive reactions were obtained in only 10 to 15 per