1. When an emulsion containing virulent Treponema pallidum is added to serum from normal rabbits and from untreated immune syphilitic rabbits that have been infected with a homologous strain of T. pallidum the mixture incubated at 37°C., and injected intracutaneously into normal rabbits, typical syphilitic lesions commonly develop at the sites of inoculation of the normal serum-spirochete mixture, while at the sites of inoculation of immune serum-spirochete mixtures usually either no lesion develops or else the incubation period of the resulting lesions is shorter and the lesions remain smaller than those produced by normal serum-spirochete mixtures.
2. In a series of preliminary experiments, of 56 areas inoculated with serum-spirochete mixtures, in 42 the suppressive action of the syphilitic serum was manifest, in 10 areas questionable evidence of protection was noted, and in 4 areas there was no evidence that the syphilitic serum had exerted a suppressive or protective action.
3. The protective action of syphilitic serum seems to have been lessened by heating to 56°C.
4. The results of the protection test in three other series of experiments were as follows: (a) Of 12 areas in 6 rabbits inoculated with normal serum-spirochete mixtures typical syphilitic lesions developed, while in the same number of areas inoculated with immune serum-spirochete mixtures there was complete or partial suppression of lesions in all. (b) Of 45 areas inoculated with serum from 10 different immune syphilitic rabbits, definite evidence of protection was observed in 37, questionable evidence in 5, and no evidence of protection in 3. (c) Of 8 areas in 4 rabbits inoculated with immune serum-spirochete mixtures no lesions developed during the period of observation, while of 8 areas in the same rabbits inoculated with one of two normal serum-spirochete mixtures typical syphilitic lesions developed in each.