The binding of heterotrimeric lymphotoxin, LT␣ 1  2 , to the LT receptor (LTR), a member of the tumor necrosis factor receptor (TNFR) superfamily, induces nuclear factor B (NF-B) activation and cell death in HT29 adenocarcinoma cells. We now show that treatment with LT␣ 1  2 or agonistic LTR antibodies causes rapid recruitment of TNFRassociated factor 3 (TRAF3) to the LTR cytoplasmic domain. Further, stable overexpression of a TRAF3 mutant that lacks the RING and zinc finger domains inhibits LTR-mediated cell death. The inhibition is specific for LTR cell death signaling, since NF-B activation by LT␣ 1  2 and Fasmediated apoptosis are not inhibited in the same cells. The mutant and endogenous TRAF3s are both recruited at equimolar amounts to the LTR, suggesting that the mutant disrupts the function of the signaling complex. These results implicate TRAF3 as a critical component of the LTR death signaling complex and indicate that at least two independent signaling pathways are initiated by LTR ligation.