Spirochetemia of syphilitic patients was demonstrated by Uhlenhuth and Mulzer (1913). Since transfusion syphilis was first reported by Fordyce (1915) and impor tance of its prevention repeatedly emphasized by Levy and Ginsburg (1927), Feldman (1928), McCluskie (1939), and others, a number of experimental and clinical studies have been made. Kast et al. (1939), Eichenlaub et al. (1941), Chin (1950), and Schmidt (1953) recommended the use of arsenic preparations for its prevention, while Nakao (1954) proposed such antibiotics as penicillin. Employing the pathogenic agents of syphilis, relapsing fever, sleeping sickness, and rat-bite fever, the authors studied the prophylactic effect of two kinds of arsenic preparations and five of antibiotics against infections. The followings are the results obtained therefrom. MATERIALS AND METHODS A single strain of Borrelia duttonii (B.d.), Trypanosoma gambiense (T.g.), Spirillum minus (S.m.) and Treponema pallidum (T.p.