The nfkb2 gene is a member of the Rel/NF-κB family of transcription factors. COOH-terminal deletions and rearrangements of this gene have been associated with the development of human cutaneous T cell lymphomas, chronic lymphocytic leukemias, and multiple myelomas. To further investigate the function of NF-κB2, we have generated mutant mice carrying a germline mutation of the nfkb2 gene by homologous recombination. NF-κB2–deficient mice showed a marked reduction in the B cell compartment in spleen, bone marrow, and lymph nodes. Moreover, spleen and lymph nodes of mutant mice presented an altered architecture, characterized by diffuse, irregular B cell areas and the absence of discrete perifollicular marginal and mantle zones; the formation of secondary germinal centers in spleen was also impaired. Proliferation of NF-κB2–deficient B cells was moderately reduced in response to lipopolysaccharide, anti-IgD-dextran, and CD40, but maturation and immunoglobulin switching were normal. However, nfkb2 (−/−) animals presented a deficient immunological response to T cell–dependent and –independent antigens. These findings indicate an important role of NF-κB2 in the maintenance of the peripheral B cell population, humoral responses, and normal spleen architecture.