The transcription factor nuclear factor (NF)-κB has been suggested to be a key mediator of the development of lymph nodes and Peyer's patches. However, targeted deletion of NF-κB/ Rel family members has not yet corroborated such a function. Here we report that when mice lacking the RelA subunit of NF-κB are brought to term by breeding onto a tumor necrosis factor receptor (TNFR)1-deficient background, the mice that are born lack lymph nodes, Peyer's patches, and an organized splenic microarchitecture, and have a profound defect in T cell–dependent antigen responses. Analyses of TNFR1/RelA-deficient embryonic tissues and of radiation chimeras suggest that the dependence on RelA is manifest not in hematopoietic cells but rather in radioresistant stromal cells needed for the development of secondary lymphoid organs.