The binding site for nuclear factor-κB (NF-κB) is present at the promoter region of the germline Cϵ gene, but there is little information on whether this factor is involved in regulating IgE synthesis by human B cells. Accordingly, we studied the role of NF-κB in germline Cϵ transcription by using two human Burkitt's lymphoma B cell lines, DND39 and DG75. In both cell lines, n-acetyl-l-cysteine (NAC), a potent thiol antioxidant, inhibited the triggering of the nuclear expression of NF-κB by IL-4 and by anti-CD40 monoclonal antibody. Although IL-4 activated signal transducers and activators of transcription (STAT) 6 in addition to NF-κB, NAC treatment or the transfection of decoy oligodeoxynucleotides for NF-κB or STAT6 only partly blocked IL-4–induced germline Cϵ transcription. However, these two decoy oligodeoxynucleotides together almost completely abrogated IL-4–induced germline Cϵ transcription. Of note, CD40-mediated enhancement of IL-4–driven germline Cϵ transcription was markedly decreased by NAC or by a decoy oligodeoxynucleotide for NF-κB. The effect of NAC was also examined on deletional switch recombination underlying the isotype switch to IgE. NAC inhibited the generation of Sμ/Sϵ switch fragments in normal human B cells costimulated with IL-4 and anti-CD40 monoclonal antibody. It also abolished IL-4–induced upregulation of CD40 but promoted upregulation of CD23. These results suggest that coordination of NF-κB and STAT6 may be required for induction of germline Cϵ transcription by IL-4, and that CD40-mediated NF-κB activation may be important in regulating both enhancement of germline Cϵ transcription and class switching to IgE. (J Allergy Clin Immunol 1997;100:S33-8.)