It has been quite generally accepted that the Pl aut\x=req-\ Vincent organisms have a pathogenic relationship to a variety of clinical entities. Unexceptionable proof of such relationship is wanting. The purpose of this report is to record clinical and experimental observations bearing on this subject. This study was undertaken at the suggestion of Dr. Joseph Brennemann.In the course of a study of a large series of diphtheria patients Plaut,1 in 1894, came on five cases of non\x=req-\ diphtheritic membranous angina, at first taken for diphtheria, in which fusiform bacilli and spirochetes were found. Plaut called these "Miller's organisms," since these had been described in 1883 by an American dentist, Willoughby D. Miller, who stated that pathogenic properties had already been ascribed to these organisms by Verneuil and Clado, who had found them in abscesses of the sublingual salivary gland, in submaxillary adenitis and in an abscess of a finger due to a human bite. Although the organisms were found beneath the gum margins in almost every mouth, and their numbers were greatly increased in gingivitis, Plaut believed them to be the causative agent in the anginas because the organisms were present in such great num¬ bers, in two instances in pure culture.In 1896, and more fully in 1899, Vincent2 described two diseases of the mucous membrane of the mouth and pharynx which he ascribed to the fusospirochetal organisms. The first was a mild stomatitis with super¬ ficial ulcérations, with a subfebrile course lasting two or three days. The ulcérations were covered with membranes, which came off easily but reformed to a less degree and gradually disappeared. In the second form a deep ulcer, which bled easily, lay beneath the membrane ; the fever was high, and a marked adenitis and a reddened edematous pharynx were present. The disease ran a protracted course, in one patient for two months. In the first form he found only the fusiform bacillus, while in the severer, ulceromembranous form From the Otho S. A. Sprague Memorial Institute Laboratory and the Children's Memorial Hospital. 1. Plaut, H. C.: Studien zur bacteriellen Diagnose der Diphtherie und den Anginen, Deutsche med. Wchnschr. 20: 920, 1894. 2. Vincent, M. H.: Recherches bact\l=e'\riologiquessur l'angine \l =a`\bacillus fusiformis, Ann. l'Inst. Pasteur 13: 609, 1899.the bacillus was associated with a spirillum in fifteen out of eighteen cases. Vincent was unable to reproduce the disease in the mouth or vagina of animals, and subcutaneous injections of the mixed organisms resulted in abscesses in which only pus organisms were found*. He was unable to cultivate the organisms and satisfac¬ torily attest their pathogenicity.Weaver and Tunnicliff3 in 1905 described the presence of the fusiform bacillus and spirillum in ulcerative stomatitis and succeeded in cultivating the bacillus anaerobically. These investigators were unable to produce any disease in rabbits with pure cultures. When mixed cultures were injected into the ears of rabbits, gangrene of the skin and deat...