To investigate the role of TNFc~ in the development of in vivo immune responses we have generated TNFoL-deficient mice by gene targeting. Homozygous mutant mice are viable and fertile, develop lymph nodes and Peyer's patches and show no apparent phenotypic abnormalities, indicating that TNFoL is not required for normal mouse development. In the absence of TNFcx mice readily succumb to L. monocytogenes infections and show reduced contact hypersensitivity responses. Furthermore, TNFot knockout mice are resistant to the systemic toxicity of LPS upon r)-galactosamine sensitization, yet they remain sensitive to high doses of LPS alone. Most interestingly, TNFcx knockout mice completely lack splenic primary B cell follicles and cannot form organized follicular dendritic cell (FDC) networks and germinal centers. However, despite the absence of B cell follicles, Ig class-switching can still occur, yet deregulated humoral immune responses against either thymus-dependent (TD) or thymus-independent (TI) antigens are observed. Complementation of TNFc~ functioning by the expression of either human or murine TNFo~ transgenes is sufficient to reconstitute these defects, establishing a physiological role for TNFc~ in regulating the development and organization of splenic follicular architecture and in the maturation of the humoral immune response.