1. Lesions of the testicle and scrotum of the rabbit produced by the treponema of syphilis and of yaws afford material for the study of chemotherapy, of serum reactions, and of immunity. 2. On experimental and clinical grounds, it appears that yaws can be cured with a smaller dose of salvarsan than is necessary to cure syphilis. 3. The spirochætæ of syphilis do not become resistant after repeated treatments with salvarsan. 4. Sodium cacodylate has little effect on the treponema of either disease. 5. Mercury, in a single intravenous dose, has a spirillicidal effect, but the necessary dose usually causes the death of the animal later from nephritis. 6. Potassium iodid, in a single intravenous dose, has no spirillicidal effect. 7. The complement fixation reaction is closely correlated with the course of an infection, as is shown by its onset and its disappearance after castration or treatment. 8. Reinfection during the course of a testicular lesion does not occur, but it occurs after regression, treatment, or extirpation. 9. Specific differences between Treponema pallidum and Trepanema pertenue can be demonstrated in infected animals treated with salvarsan and reinoculated.