Transcription factors NF-kappaB1 and c-Rel, individually dispensable during embryogenesis, serve similar, yet distinct, roles in the function of mature hemopoietic cells. Redundancy among Rel/NF-kappaB family members prompted an examination of the combined roles of c-Rel and NF-kappaB1 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 G(1) 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-kappaB1 and c-Rel appear to be restricted to regulating the activation and function of mature cells.