Members of the tumor necrosis factor (TNF)-nerve growth factor (NGF) receptor family have been shown to be important costimulatory molecules for cellular activation. 4-1BB and Ox40 are two recently described members of this protein family which are expressed primarily on activated T cells. To gain insight into the signaling pathways employed by these factors, yeast two-hybrid library screens were performed with the cytoplasmic domains of 4-1BB and Ox40 as baits. TNF receptor-associated factor 2 (TRAF2) was identified as an interacting protein in both screens. The ability of both 4-1BB and Ox40 to interact with TRAF2 was confirmed in mammalian cells by coimmunoprecipitation studies. When the binding of the receptors to other TRAF proteins was investigated, 4-1BB and Ox40 displayed distinct binding patterns. While 4-1BB bound TRAF2 and TRAF1, Ox40 interacted with TRAF3 and TRAF2. Using deletion and alanine scanning analysis, we defined the elements in the cytoplasmic domains of both receptors that mediate these interactions. The 4-1BB receptor was found to have two independent stretches of acidic residues that can mediate association of the TRAF molecules. In contrast, a single TRAF binding domain was identified in the cytoplasmic tail of Ox40. The cytoplasmic domains of both receptors were shown to activate nuclear factor κB in a TRAF-dependent manner. Taken together, our results indicate that 4-1BB and Ox40 bind TRAF proteins to initiate a signaling cascade leading to activation of nuclear factor κB.