T cell-mediated adaptive immunity is required to help clear infection with the facultative intracellular bacterial pathogen Salmonella enterica serovar Typhimurium (S. Typhimurium), yet development of T cell-mediated adaptive immunity to S. Typhimurium has been described as slow and inefficient. A key step in inducing T cell-mediated adaptive immunity is T cell priming; the activation, proliferation, and differentiation of naive T cells following initial encounter with Ag. We previously demonstrated that S. Typhimurium had a direct inhibitory effect on naive T cells from mouse, blocking their proliferation. In this study, we show that S. Typhimurium down-modulates expression of the TCR β-chain, a molecule that is essential for Ag recognition and T cell function. Specifically, we demonstrate that reduced amounts of surface and intracellular TCR-β protein and decreased levels of tcrβ transcript are expressed by T cells cultured in the presence of S. Typhimurium. We further show that the down-modulation of TCR-β expression requires contact between S. Typhimurium and the T cells and that once contact occurs, a factor capable of reducing TCR-β expression is secreted. These results provide new insight into the mechanism by which S. Typhimurium may inhibit T cell priming and avoid clearance by the adaptive immune system.