TRAF5 [tumor necrosis factor (TNF) receptor-associated factor 5] is implicated in NF-kappaB and c-Jun NH(2)-terminal kinase/stress-activated protein kinase activation by members of the TNF receptor superfamily, including CD27, CD30, CD40, and lymphotoxin-beta receptor. To investigate the functional role of TRAF5 in vivo, we generated TRAF5-deficient mice by gene targeting. Activation of either NF-kappaB or c-Jun NH(2)-terminal kinase/stress-activated protein kinase by tumor necrosis factor, CD27, and CD40 was not abrogated in traf5(-/-) mice. However, traf5(-/-) B cells showed defects in proliferation and up-regulation of various surface molecules, including CD23, CD54, CD80, CD86, and Fas in response to CD40 stimulation. Moreover, in vitro Ig production of traf5(-/-) B cells stimulated with anti-CD40 plus IL-4 was reduced substantially. CD27-mediated costimulatory signal also was impaired in traf5(-/-) T cells. Collectively, these results demonstrate that TRAF5 is involved in CD40- and CD27-mediated signaling.