The ultimate objective of this work is the development of nine-banded armadillos into a suitable model for the study of leprosy as it occurs in human beings. It is first necessary to develop a susceptibility test to infection with Mycobacterium leprae in uninfected armadillos and to find the infectious dose to which 90 percent of the armadillos are resistant. The susceptibility test is based on the capability of vaccinated armadillos to develop delayed-type hypersensitivity to M. leprae-protein as shown by dermal response and lymphocyte transformation. The suitable infectious dose will be determine from the results of infections with decreasing numbers of M. leprae.