The genus Leptotrichia was omitted from Bergey's iManual of Determinative Bacteriology (Breed, Murray, and Smith, 1957) because a wide variety of unrelated, oral filamentous microorganisms had been previously placed in this genus. The confusion concerning the classification and definition of this genus has been emphasized by Baird-Parker and Davis (1958) and Gilmour and Hunter (1958). Two very different organisms have been termed Leptotrichia buwcalis. According to Baird-Parker and Davis (1958), the first of these was described by several early investigators as a grampositive, fusiform, anaerobic bacterium found in materia alba of the teeth. The cultural and biochemical reactions of this microorganism have been studied by Thj0tta, Hartman, and B0e (1939), B0e and Thj0tta (1944), and Hamilton and Zahler (1957). The second organism, a gram-positive, filamentous, facultative actinomycete, was described and studied by Bibby and Berry (1939) and Bartels (1943). More recently, Gilmour and Hunter (1958) and Howell and Rogosa (1958) reported improved techniques for the isolation and maintenance of these bacteria. To resolve the confusion which resulted from the application of the name Leptotrichia buccalis to both of these organisms, Davis and Baird-Parker (1959) suggested that two species be recognized in the genus Leptotrichia. They refer to the first type (the fusiform bacillus) as Leptotrichia buccalis, and the second type (the actinomycete) as Leptotrichia dentium. This paper is concerned with a study of the oral filamentous organism which has been designated recently as Leptotrichia dentium. Biochemical reactions were investigated to be certain we were dealing with the same organism described by other workers. Agglutination, cross-agglutination, and precipitin reactions were undertaken

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