Transcription factors NF-κB1 and c-Rel, individually dispensable during embryogenesis, serve similar, yet distinct, roles in the function of mature hemopoietic cells. Redundancy among Rel/NF-κB family members prompted an examination of the combined roles of c-Rel and NF-κB1 by using mice that lack both proteins. Embryonic development and the maturation of hemopoietic progenitors were unaffected in nfkb1^−/−c-rel^−/− mice. Peripheral T cell populations developed normally, but follicular, marginal zone, and CD5⁺ peritoneal B cell populations all were reduced. In culture, a failure of mitogen-stimulated nfkb1^−/−c-rel^−/− B cells to proliferate was caused by a cell cycle defect in early G₁ that prevented growth. In vivo, defects in humoral immunity and splenic architecture seen in nfkb1^−/− and c-rel^−/− mice were exacerbated in the double mutant mice. These findings demonstrate that in the B lineage overlapping roles for NF-κB1 and c-Rel appear to be restricted to regulating the activation and function of mature cells.