We have previously reported that peripheral blood gamma/delta + T cells proliferate in high frequency (1 in 2-20) in response to heat-killed Mycobacterium tuberculosis (M.tb.). In the present study, the T cell receptor phenotype of mycobacteria-responsive human gamma/delta + T cells was analyzed in primary cultures with a set of monoclonal antibodies (mAbs) directed against V gamma 9, V delta 1, and V delta 2. When unseparated T cells were stimulated with M.tb., all proliferating gamma/delta + T cells expressed V gamma 9 (and V delta 2) after culture. Selective depletion of V gamma 9-bearing cells before culture completely abolished the proliferative response of all gamma/delta + cells (but did not inhibit reactivity of alpha/beta + T cells). In addition, when CD4- CD8- thymocytes were stimulated with M.tb., there was again selective outgrowth of V gamma 9+ cells. In this case, the starting responder population contained few (0.5-1.8%) V gamma 9+ and many (11.5-31.5%) V delta 1+ cells that did not coexpress V gamma 9. These V delta 1+ cells were not activated by M.tb. but could be readily stimulated by anti-V delta 1 mAb A13. Finally, a V gamma 9-specific mAb selectively suppressed the proliferative response of gamma/delta + T cells to M.tb. Taken together, our results demonstrate that, within gamma/delta + T cells, reactivity towards M.tb. is an exclusive property of V gamma 9+/V delta (2+)-bearing cells.