Mice with a targeted disruption of the Rel/nuclear factor-kappaB family member RelB develop a complex inflammatory phenotype, myeloid hyperplasia, and splenomegaly due to extramedullary hemopoiesis. In this work, we report that RelB-deficient mice, in addition to the pathologic changes, were highly susceptible to infection by the facultative intracellular bacterium Listeria monocytogenes. RelB binds transcriptionally active kappaB motifs in the TNF-alpha promoter in normal cells, and in vitro studies with macrophages isolated from RelB-deficient animals revealed impaired production of TNF-alpha in response to LPS and IFN-gamma. RelB-deficient mice also were unable to mount a protective immune response against lymphocytic choriomeningitis virus. These results indicate a defective T cell-macrophage interaction and cytotoxic T cell response, respectively, in mice lacking RelB. Analysis of resting and specific Ab production demonstrated that while RelB is not required for the secretion of Ig isotypes that result from heavy chain class switching, it is necessary for normal production of Ag-specific IgG in response to T cell-dependent and -independent stimuli. Thus, RelB is not only essential for a normal hemopoietic system in the unchallenged animal, but also involved in various specific and nonspecific immune responses.