Despite the availability of drugs effective in producing a bacteriological cure, tuberculosis presents continuing problems in its control, especially in the developing world. An effective immunotherapy to be used with chemotherapy is urgently required. Intradermal injection of a suspenison of killed Mycobacterium vaccae switches off the tissue-necrotizing component of the Koch phenomenon, and promotes cell-mediated responses to the common, putative protective, mycobacterial antigens. These properties led to the empolyment of the suspension in immunotherapy as an adjunct to chemotherapy in the treatment of both tuberculosis and leprosy. The evidence leading to these conclusions is reviewed.