The work on the cultivation of the noncultivated mycobacteria will go forward in three directions. A. MODEL SYSTEM: The further use of Mycobacterium lepraemurium (agent of rat leprosy and the "obligate intracellular" microbe which we have been cultivating) as a model for Mycobacterium leprae (agent of human leprosy) to explore further principles for identifying growth factors for host dependent microbes. This work will improve our comprehension of how to compensate the less adequate metabolic and biosynthetic pathways in M. leprae. B. THE HEISER PROJECT: Dr. Kvach and I are undertaking to accelerate the growth of M. leprae in experimental animals by means of increasing the in vivo availability of iron and selected substrates. It is hoped that this work will facilitate the use of animals as a fundamental tool in many lines of leprosy research. Ultrasensitive determinations of adenosine triphosphate (ATP) are employed to be certain that the M. leprae cells possess maximal growth potential and infectiousness. C. CULTIVATION OF M. LEPRAE: The lessons learned during the cultivation of the model species will be applied to see if we can maintain integrated metabolic systems while the in vivo-grown cells fabricate in vitro-type cell membranes. If and when this can be accomplished, factors that facilitate growth in bacteriologic media will be investigated.