Signaling by the p55 tumor necrosis factor (TNF) receptor and by the structurally related receptor Fas/ APO1 is initiated by receptor clustering. Data presented here and in other recent studies (Wallach, O., Boldin, M., Varfolomeev, E. E., Bigda, Y., Camonis, H. J. and Mett, I. (1994) Cytokine 6, 556; Song, H. Y., Dunbar, J. D., and Bonner, D. B. (1994) J. Biol. Chem. 269, 22492-22495) indicate that part of that region within the intracellular domains of the two receptors that is involved in signaling for cell death, as well as for some other effects (the "death domain"specifically self-associates. We demonstrate also the expected functional consequence of this association; a mere increase in p55 TNF receptor expression, or the expression just of its intracellular domain, is shown to trigger signaling for cytotoxicity as well as for interleukin 8 gene induction, while expression of the intracellular domain of Fas/APO1 potentiates the cytotoxicity of co-expressed p55 TNF receptor. These findings indicate that the p55 TNF and Fas/APO1 receptors play active roles in their own clustering and suggest the existence of cellular mechanisms that restrict the self-association of these receptors, thus preventing constitutive signaling.