Despite many studies, the regulation of CD4 + T cell apoptosis during the shutdown of immune responses is not fully understood. We have investigated the molecular mechanisms of IFN- γ in regulating apoptosis of CD4 + T cells during bacillus Calmette-Guérin (BCG) infection of mice. Our data provide new insight into the regulation of CD4 + T cell apoptosis by IFN- γ . As CD4 + T cells responded to BCG infection, there was a coordinated increase in IFN- γ production by effector CD4 + T cells and a coordinated IFN- γ -dependent up-regulation of many diverse apoptosis-pathway genes in effector CD4 + T cells. Unexpectedly, IFN- γ up-regulated transcripts and protein expression of Bcl-2, Bax, Bim, Bid, Apaf-1, and caspase-9 in activated CD4 + T cells—components of the apoptosis machinery that are involved in promoting mitochondrial damage-mediated apoptosis. Wild-type, but not IFN- γ knockout, CD4 + T cells underwent apoptosis that was associated with damaged mitochondrial membranes. IFN- γ also up-regulated expression of cell-extrinsic signals of apoptosis, including TRAIL, DR5, and TNFR1. Cell-extrinsic apoptosis signals from TNF- α , TRAIL, and NO were capable of damaging the mitochondrial membranes in activated CD4 + T cells. Moreover, activated CD4 + T cells from BCG-infected DR5, TNFR1, and inducible NO synthase knockout mice had impaired caspase-9 activity, suggesting impaired mitochondria-pathway apoptosis. We propose that IFN- γ promotes apoptosis of CD4 + T cells during BCG infection as follows: 1) by sensitizing CD4 + T cells to apoptosis by inducing intracellular apoptosis molecules and 2) by inducing cell-extrinsic apoptosis signals that kill CD4 + effector T cells.