Mice deficient in LTα (Lα(-/-)) lack lymph nodes and Peyer's patches. This action of LTα in lymph node organogenesis appears to be mediated by the membrane form of LT using a mechanism independent of TNF receptor I (TNFR-I) or II (TNFR-II). In contrast, normal Peyer's patch development appears to require both LTα and TNFR-I, with TNFR-I(-/-) mice showing hypoplastic Peyer's patch structures. LTα(-/-) mice also fail to support the normal segregation of T-cell and B-cell zones within the splenic white pulp. Again, this occurs via a mechanism independent of TNFR-I or TNFR-II. Additionally, follicular dendritic cell (FDC) clusters or germinal centers fail to develop in the spleen of LTα(-/-) animals. Mice deficient in either TNFα or TNFR-I also fail to develop splenic FDC clusters and germinal centers, indicating that signaling by both LTα and TNFα is required for development of these specialized lymphoid tissue structures. Finally, the splenic white pulp areas in LTα(-/-) mice lack the marginal zone of monoclonal antibody MOMA-1-staining metallophilic macrophages, whereas TNFR-I-deficient mice have preserved MOMA-I staining. Thus, certain actions of LTα to regulate spleen white pulp architecture are mediated by receptors other than TNFR-I, most likely by the LTβR or a closely related receptor. We tested whether germinal centers are essential for maturation of T-cell-dependent antibody responses. When LTα(-/-) mice were immunized with low doses of NP-ovalbumin (NP-OVA) adsorbed to alum, there was dramatically impaired production of high affinity anti-NP IgG; however, after immunization with high doses of NP-OVA adsorbed to alum, LTα(-/-) mice mounted a high affinity NP-specific serum IgG response similar to wild-type mice, all in the absence of germinal centers or clustered FDC. Thus, although germinal centers enhance the processes required for maturation of the humoral immune response, the mechanisms responsible for affinity maturation are not absolutely dependent on the presence of germinal centers.