Leprosy is of interest to immunologists because the varied clinical manifestations of the disease correlate closely with the immunological spectrum. Resistance to infection is dependent on appropriate cell-mediated immunity, but patients with the lepromatous form fail to respond to antigens of M. leprae. In vitro studies have revealed the existence of T-suppressor cells of the phenotype CD8+, CD3+, HLA-DR+, FcR+, 9.3-, which are restricted by major histocompatibility complex (MHC) class II antigens. Several new candidate vaccines against leprosy have been effective in breaking immunological unresponsiveness and engendering cell-mediated immunity in lepromatous leprosy patients, including the combination of BCG+ killed M. leprae. Because BCG has unique adjuvant properties, we have begun to use molecular genetic approaches to develop BCG into a multivaccine vehicle capable of immunizing simultaneously against several pathogens. Both phage-based and plasmid-based strategies have been successfully developed for introducing selectable markers into BCG for the first time.