The lungs are considered to have an impaired capacity to contain infection by pathogenic mycobacteria, even in the presence of effective systemic immunity. In an attempt to understand the underlying cellular mechanisms, we characterized the γδ T cell population following intranasal infection with Mycobacterium bovis bacillus Calmette-Guérin (BCG). The peak of γδ T cell expansion at 7 days postinfection preceded the 30 day peak of αβ T cell expansion and bacterial count. The expanded population of γδ T cells in the lungs of BCG-infected mice represents an expansion of the resident Vγ2 T cell subset as well as an influx of Vγ1 and of four different Vδ gene-bearing T cell subsets. The γδ T cells in the lungs of BCG-infected mice secreted IFN-γ following in vitro stimulation with ionomycin and PMA and were cytotoxic against BCG-infected peritoneal macrophages as well as against the uninfected J774 macrophage cell line. The cytotoxicity was selectively blocked by anti-γδ TCR mAb and strontium ions, suggesting a granule-exocytosis killing pathway. Depletion of γδ T cells by injection of specific mAb had no effect on the subsequent developing CD4 T cell response in the lungs of BCG-infected mice, but significantly reduced cytotoxic activity and IFN-γ production by lung CD8 T cells. Thus, γδ T cells in the lungs might help to control mycobacterial infection in the period between innate and classical adaptive immunity and may also play an important regulatory role in the subsequent onset of αβ T lymphocytes.