Termination of an immune response requires elimination of activated T lymphocytes by activation-induced cell death (AICD). In AICD, CD95 (Apo-1/Fas) ligand (L) triggers apoptosis of CD95-positive activated T lymphocytes. In AIDS patients, AICD is strongly enhanced and accelerated. We and others have previously shown that HIV-1 trans-activator of transcription (HIV-1 Tat) sensitizes T cells toward CD95-mediated apoptosis and up-regulates CD95L expression by affecting the cellular redox balance. In this study, we show that it is hydrogen peroxide (H 2 O 2) that functions as an essential second messenger in TCR signaling. The H 2 O 2 signal combined with simultaneous calcium (Ca 2+) influx into the cytosol constitutes the minimal requirement for induction of CD95L expression. Either signal alone is insufficient. We further show that HIV-1 Tat interferes with TCR signaling and induces a H 2 O 2 signal. H 2 O 2 generated by HIV-1 Tat combines with CD4-dependent calcium influx and causes massive T cell apoptosis. Thus, our data provide an explanation for CD4+ T lymphocyte depletion during progression of AIDS.