Activation-induced cell death is a process by which overactivated T cells are eliminated, thus preventing potential autoimmune attacks. Two known mediators of activation-induced cell death are Fas (CD95) ligand (FasL) and APO2 ligand (APO2L)/TNF-related apoptosis-inducing ligand (TRAIL). We show here that upon mitogenic stimulation, bioactive FasL and APO2L are released from the T cell leukemia Jurkat and from normal human T cell blasts as intact, nonproteolyzed proteins associated with a particulate, ultracentrifugable fraction. We have characterized this fraction as microvesicles of 100–200 nm in diameter. These microvesicles are released from Jurkat and T cell blasts shortly (≤ 1 h) after PHA stimulation, well before the cell enters apoptosis. FasL-and APO2L-containing vesicles are also present in supernatants from PHA-activated fresh human PBMC. These observations provide the basis for a new and efficient mechanism for the rapid induction of autocrine or paracrine cell death during immune regulation.