CD40 plays a critical role in immunoglobulin (Ig) class switching in B cells, but the molecular events involved remain poorly understood. Using CD40 mutants with impairments in their ability to bind selected TNF receptor-associated factor (TRAF) family proteins, we observed that CD40-mediated transcriptional induction of the germ-line Ig-Cgamma1- and Ig-Cepsilon promoters was markedly reduced by mutations that prevent TRAF2, TRAF3, TRAF5 or TRAF6 binding. Moreover, co-expression of trans-dominant inhibitory forms of TRAF2, 3, 5 or 6 with wild-type CD40 also suppressed induction of these promoters. Overexpression of TRAF2 or TRAF6 was sufficient to induce transcription of the C(H) promoters through an NF-kappaB-dependent mechanism. In contrast, TRAF3 and TRAF5 failed to induce these promoters, implying a more indirect role for these TRAF family members. Altogether, the results demonstrate a non-redundant role for multiple TRAF in the signal transduction pathways by which CD40 induces transcription of germ-line C(H) promoters. Since C(H) germ-line transcription represents an obligatory step in Ig class switching in B cells, these findings suggest that interference with the functions of any of these TRAF might provide a means of preventing class switching for therapeutic purposes.