Tumor necrosis factor (TNF) family proteins are essential regulators of life and death in the immune system and bind to adaptor proteins called TNFR associated factors (TRAFs). TRAF2 acts downstream of several TNFRs and links them to survival. TRAF1 appears to function as a modulator of TRAF2. To date, there have been conflicting results as to whether TRAF1 plays a positive or negative role in signaling by TNFRs. To elucidate the role of TRAF1, we compared the immune response to influenza virus in WT versus TRAF1-/- mice. TRAF1-/- mice exhibit decreased expansion of Db/NP366--374 specific CD8 T cells. Furthermore, TRAF1-/- T cells showed reduced IL-2 production and reduced Bcl-XL upregulation in response to anti-CD3 plus 4-1BB signaling and were also more prone to cell death in culture. Overall, the results show that TRAF1 in both T cells and non-T cells is a positive regulator of the T cell response.