This report describes the in vitro proliferative responses of peripheral blood γδ T cells to defined mycobacterial protein Ags and the immunomodulatory effect of γδ T cells in cattle infected with Mycobacterium bovis. γδ T cell responses were specific to M. bovis infection because they were detected in cattle either experimentally or naturally infected with M. bovis, but were not present in uninfected controls. Proliferating γδ T cell cultures produced enhanced levels of IFN-γ and TGF-β, but not IL-2 in response to the more immunodominant mycobacterial Ags. Depletion of γδ T cells from PBMC resulted in an increased Ag-specific proliferation in half the animals tested, indicating a suppressive effect of γδ T cells upon other (αβ) T cell responses. Because γδ T cells constitute a major T cell population in the peripheral blood of cattle, the activities of γδ T cells described in this report could make a significant contribution to the immune response in bovine tuberculosis. Tuberculosis is currently increasing in the British herd. As a result of this, the need for new vaccines and diagnostic reagents was highlighted in a recent independent scientific review (1). For the rational development of such reagents it is useful to understand something of the underlying immune mechanisms that constitute this immunopathological disease. In this report we concentrate upon the activity of the γδ T cell and speculate upon its role in bovine tuberculosis. γδ T cells were identified >15 years ago (2) and yet the physiological role of these cells and their ligands is still not fully understood. They have been shown to accumulate in the infected tissues both in mice (3) and humans (4) infected with M. tuberculosis. In mice, there are conflicting reports as to whether γδ T cells offer protective immunity (5), or not (6) against M. tuberculosis and M. bovis bacillus Calmette-Guérin (BCG),3 respectively. Other reports identify a possible role for γδ T cells in cellular trafficking following infection with M. tuberculosis (7), or in the early establishment of the cytokine profile following infection by a pathogen (8). In humans γδ T cells have been shown to be increased in the blood of anergic, skin-test-negative tuberculosis patients (9), suggesting a suppressive role of γδ T cells in these individuals. Very few ligands for γδ T cells have been identified (10). Well-defined mycobacterial ligands for γδ T cells include the heat shock protein (hsp) 65 in both mice and humans (11, 12), and nonpeptide Ags in humans (13). Other reports have described γδ reactivity to as yet unknown low molecular weight protein Ags of M. tuberculosis in humans (14, 15). Cattle and other ruminants, unlike humans and mice (16, 17), have large numbers of γδ T cells in the peripheral blood (18, 19, 20). At least two distinct subpopulations of γδ T cells have been identified based on the mutually exclusive expression of the surface markers WC1, CD2, and CD8, and these subpopulations are uniquely distributed in various tissues (21). In bovine peripheral blood the majority of γδ T cells express WC1, with the phenotype WC1+CD2−CD8−, while a small population of γδ T cells are WC1-negative, but express CD2 and, largely, CD8 (WC1−CD2+CD8+/−) (22). Bovine peripheral blood γδ T cells have been shown to proliferate in response to Theileria-infected cells in the presence of IL-2 (23) and down-regulate T cell responses in bovine Fasciola infection (24). γδ T cell lines have also been demonstrated to be effective Ag-presenting cells for CD4+ T cells, causing their proliferation (25). In this investigation of bovine tuberculosis, we have asked two questions: 1) do bovine peripheral blood γδ T cells respond to mycobacterial protein Ags; and 2) what effect, if any, do γδ T cells have on the activity of other T cells. This report shows that bovine γδ T cells can both proliferate and enhance cytokine production in response to protein Ags, and describes an immunoregulatory function of γδ T cells that can suppress Ag-specific responses of other (αβ) T cells.