The lepromatous leprosy granuloma is a dynamic entity requiring a steady influx of macrophages (Mφ) for its maintenance. We have developed an in vitro model to study the fate of Mycobacterium leprae in a LL lesion, with and without immunotherapeutic intervention. Target cells, consisting of granuloma Mφ harvested from the footpads of M. leprae-infected athymic nu/nu mice, were cocultured with normal or IFN-γ-activated (ACT) effector Mφ. The bacilli were recovered and assessed for viability by radiorespirometry. M. leprae recovered from target Mφ possessed high metabolic activity, indicating a viable state in this uncultivable organism. M. leprae recovered from target Mφ incubated with normal effector Mφ exhibited significantly higher metabolism. In contrast, bacilli recovered from target Mφ cocultured with ACT effector Μφ displayed a markedly decreased metabolic activity. Inhibition by ACT Mφ required an E: T ratio of at least 5: 1, a coculture incubation period of 3–5 days, and the production of reactive nitrogen intermediates, but not reactive oxygen intermediates. Neither IFN-γ nor TNF-α were required during the cocultivation period. However, cell-to-cell contact between the target and effector Mφ was necessary for augmentation of M. leprae metabolism by normal effector Mφ as well as for inhibition of M. leprae by ACT effector Mφ. Conventional fluorescence microscopy and confocal fluorescence microscopy revealed that the bacilli from the target Mφ were acquired by the effector Mφ. Thus, the state of Mφ infiltrating the granuloma may markedly affect the viability of M. leprae residing in Mφ in the lepromatous lesion.