1. Rabbits can be infected in the testicle with the spirochetæ of yaws as well as with those of syphilis and the infection can be continued through successive generations in pure culture.
2. The infection shows itself by enlargement of the testicle and the presence of a nodule varying in size from that of a pea to that of an olive. The infection consists in a necrosis of the tubules, an infiltration of round cells and the new formation of an edematous connective tissue.
3. The complement fixation reaction occurs in rabbits infected with the spirochetæ of yaws as well as in those infected with the spirochetæ of syphilis.
4. This lesion makes possible the investigation of the problems of cultivation, of immunity and of treatment.