1. The skin lesions in rabbits and guinea pigs following intradermal injection of tubercle bacilli (5 strains) were greatly increased in size and severity when testicle extract was added to the inoculum. Such enhancement was followed by a more widespread and rapidly progressing disease only when virulent strains were employed.
2. Attempts to suppress the development of skin lesions resulting from the injection of either normal or tuberculous rabbits with very small quantities of tubercle bacilli mixed with testicle extract were unsuccessful.
3. The skin reactions of tuberculous guinea pigs to tuberculo-protein MA 100 were greatly increased in size and markedly reduced in intensity by the addition of testicle extract to the protein solution. The toxic effect of larger quantities of tuberculo-protein was not altered by the addition of testicle extract.
4. The dispersion of tubercle bacilli through the skin of tuberculous rabbits resulted in a marked enhancement of the Koch phenomenon but was not followed by any extension of the new infection to the viscera. Tuberculous rabbits infected on two occasions with dead tubercle bacilli suspended in testicle extract showed an increased resistance to the disease when compared with controls receiving dead bacilli suspended in saline solution.
5. The resistance conferred upon tuberculous guinea pigs by superinfection was greatly increased when the bacilli employed were dispersed through the skin with testicle extract.
6. The parenteral administration of large quantities of testicle extract to recently infected guinea pigs did not result in any increase in the extent of the visceral lesions.
7. The partial immunity conferred upon guinea pigs and rabbits by vaccination with heat-killed tubercle bacilli was increased as a result of dispersion of the vaccine through the skin with testicle extract.