In 1913 one of us (1) first described agglutination of a pure culture of Treponema pallidum by the sera of rabbits injected with a living and heat-killed culture.

Summary The results of this study conducted with one culture of treponema pallidum may be summarized as follows: Fresh active normal serum exerts no treponemicidal action on culture pallida in vitro in the proportion of one or two parts of serum to one part of diluted fluid culture.The sera of persons in the tertiary and latent stages of syphilis exert no appreciable treponemicidal action on culture pallida in vitro under similar conditions.The sera of syphilitic persons yielding typical luetin reactions did not exert any treponemicidal activity in vitro.There is no direct relation between the occurrence of cutaneous hypersensitiveness to luetin and the presence of agglutinin for culture pallida in the blood serum.There is no direct relation between the occurrence of cutaneous hypersensitiveness to luetin and the presence of a complement-fixing antibody with an antigen of luetin in the blood serum; both the anaphylactic and complement-fixing antibodies were present or absent together in 52 per cent of cases, while the former was present and the latter absent in 40 per cent. The anaphylactic and complement-fixation reactions indicate an infection with treponema pallidum, but as based upon experiments in vitro neither can be regarded as indicating the co-existence of a treponemolysin or as an index of immunity to syphilis.The sera of a larger percentage of syphilitics contain agglutinin for treponema pallidum than complement-fixing antibody when luetin is used as antigen. It is probable that this difference is due in large part to the poor antigenic sensitiveness of luetin as antigen; when ordinary lipoidal extracts were used as antigens both complement fixation and agglutination occurred together with the majority of sera.As based mainly upon the results of these treponemicidal and other tests in vitro, the anaphylactic luetin test cannot be regarded as an index of resistance to treponema pallidum; these experiments and the failure of others (Neisser and Bruck; Nakano and Noguchi) to produce active immunity in animals to syphilis indicate that a lytic type of immunity is absent in syphilis or, at least, that it plays but a minor rôle in this infection.