B and T lymphocyte attenuator (BTLA), like its relative programmed cell death-1 (PD-1), is a receptor that negatively regulates murine T cell activation. However, its expression and function on human T cells is currently unknown. We report in this study on the expression of BTLA in human T cell subsets as well as its regulation on virus-specific T cells during primary human CMV infection. BTLA is expressed on human CD4+ T cells during different stages of differentiation, whereas on CD8+ T cells, it is found on naive T cells and is progressively downregulated in memory and differentiated effector-type cells. During primary CMV infection, BTLA was highly induced on CMV-specific CD8+ T cells immediately following their differentiation from naive cells. After control of CMV infection, BTLA expression went down on memory CD8+ cells. Engagement of BTLA by mAbs blocked CD3/CD28-mediated T cell proliferation and Th1 and Th2 cytokine secretion. Finally, in vitro blockade of the BTLA pathway augmented, as efficient as anti–PD-1 mAbs, allogeneic as well as CMV-specific CD8+ T cell proliferation. Thus, our results suggest that, like PD-1, BTLA provides a potential target for enhancing the functional capacity of CTLs in viral infections.