A NUMBER of murine T-cell hybridomas undergo apoptosis within a few hours of activation by specific antigens, mitogens, antibodies against the T-cell antigen receptor, or a combination of phorbol ester and calcium ionophore^1–3. This phenomenon has been extensively studied as a model for clonal deletion in the immune system, in which potentially autoreactive T cells eliminate themselves by apoptosis after activation, either in the thymus⁴ or in the periphery⁵. Here we show that the Fas/CD95 receptor, which can transduce a potent apoptotic signal when ligated^6,7, is rapidly expressed following activation of T-cell hybridomas, as is its functional, membrane-bound ligand⁸. Interference with the ensuing Fas/Fas-ligand interaction inhibits activation-induced apoptosis. Because T-cell receptor ligation can induce apoptosis in a single T hybridoma cell, we suggest that the Fas/Fas-ligand interaction can induce cell death in a cell-autonomous manner.