Experiments will be carried out in pursuit of the original objectives of this research: 1. to screen drugs for activity against Mycobacterium leprae in mice; 2. to carry out pharmacologic studies in mice and other species, of drugs previously shown to be active; and 3. to carry out studies of M. leprae itself and of host-parasite relationships in the M. leprae-infected mouse. Among the studies to be carried out are the following: 1. immunologic studies of delayed-type hypersensitivity and lymphocyte stimulation in response to lepromins and leprolins in mice infected with M. leprae; 2. studies of activated macrophages from the foot pads of mice similarly infected; 3. studies of mice lymphadenectomized before and during infection with M. leprae; 4. studies concerning synergy between the dihydrofolate reductase inhibitors and dapsone in M. leprae-infected mice; 5. studies intended to perfect and utilize a bioassay for rifampin in conjunction with a recently developed chemical assay for this drug. BIBLIOGRAPHIC REFERENCES: T.M. Welch, L. Levy. Preliminary studies with the lymph node lymphocytes of Mycobacterium leprae-infected mice. Int. J. Lep. 44: 199 (1976). J.L. Krahenbuhl, T.M. Welch, L. Levy. The presence of systemic and local activated macrophages in mice infected in the foot pad with Mycobacterium leprae and Mycobacterium marinum. Int. J. Lep. 44: 206 (1976).