RelB, a member of the NF-κB family of transcription factors, is essential for dendritic cell (DC) maturation. Recent findings indicate that RelB is exclusively regulated through its interaction with cytoplasmic NF-κB2/p100. The studies presented in this report show that DCs lacking NF-κB2 have dramatically enhanced RelB activity, associated with increased MHC class II and costimulatory molecule expression and an enhanced ability to induce CD4+ T cell responses. These studies identify a novel role for NF-κB2 in the negative regulation of RelB-induced DC maturation, with critical consequences for the regulation of adaptive immune responses.