T-cell clones were established from Mycobacterium tuberculosis-immunized mice. These clones had the phenotype Thy-1+ L3T4+ Lyt-2- and were restricted by the H-2I-A locus. After antigen stimulation, the T-cell clones secreted interleukin-2 and gamma interferon. Factors produced by these T-cell clones activated normal bone marrow macrophages for antimycobacterial activity in vitro. Furthermore, the T-cell clones could adoptively confer delayed-type hypersensitivity on normal recipient mice. These findings indicate that the T-cell clones clones expressed relevant functions of antimycobacterial immunity. The antigen reactivity of the T-cell clones to different mycobacterial species ranged from broad cross-reactivity to stringent specificity, and none of the clones distinguished between M. tuberculosis and M. bovis. Thus, M. tuberculosis-immune helper/inducer T cells of identical phenotype, genetic restriction, and function varied in their antigen specificity. T-cell clones of the type described will facilitate functional characterization of mycobacterial antigens on the T-cell level.