1,4-Dimefhyl-7-isopropyl-bicyclo-decapentane (BD.I) has been shown to ameliorate the course of experimental tuberculosis of guinea pigs. Further experiments demonstrate that tuberculin reactions in BD.I treated animals, whether normal or immunized, were markedly milder than in similar but untreated groups of animals. Tuberculous lesions in BD.I treated normal and immunized guinea pigs were less severe when compared with untreated animals as seen at autopsy 6, 8, and 12 weeks after virulent superinfection. It seems that the drug does not interfere with the immune principle. Partial desensitization and the therapeutic effects are attributed to stimulation of the cellular defense apparatus.