Engagement of CD40 by its ligand induces IKK and mitogen-activated protein kinase (MAPK) phosphorylation and transcriptional activation, leading to activation and differentiation of B cells. These events are most likely transduced by adaptor molecules that are recruited to the CD40 cytoplasmic domain, called TNF receptor-associated factors (TRAF). We have engineered a chimeric CD40 molecule using the human extracellular sequence and the murine cytoplasmic domain to assess the contribution that specific TRAF binding domains provide to the cytoplasmic signaling functions of CD40. The data presented here show that the shared binding site for TRAF2 and TRAF3 accounts for receptor internalization, and the majority of signaling through CD40, but is redundant with the TRAF6 binding site for activation of p38 and NFkappaB signaling pathways. Disruption of the TRAF2/3 binding site results in a delayed and diminished kinase pathway induction, but complete preclusion of all signals requires the disruption of more than the two known TRAF binding sites. The specific TRAF dependency of CD40-induced growth arrest, TNF-alpha production, and phosphorylation of signaling molecules are shown, while p38 MAPK activation and cell surface antigen modulation suggest TRAF independent CD40 signaling in B cells.