Inactivation of genes encoding members of TNF and TNF receptor families reveal their divergent roles in the formation and function of secondary lymphoid organs. Most lymphotoxin alpha (ltalpha)- and all lymphotoxin beta receptor (ltbetar)-deficient mice are completely devoid of lymph nodes (LNs); however, most lymphotoxin beta (ltbeta)-deficient mice develop mesenteric LNs. Tnf- and tnfrp55-deficient mice develop a complete set of LNs, while ltbeta/tnfrp55 double-deficient mice lack all LNs, demonstrating cooperation between LTbeta and TNFRp55 in LN development. Now we report that ltbeta/tnf double-deficient mice develop the same set of mucosal LNs as do ltbeta-deficient mice, suggesting that ligands other than TNF signal through TNFRp55 during LN development. These LNs retain distinct T and B cells areas; however, they lack follicular dendritic cell networks. Structures resembling germinal centers can be found in the LNs from immunized ltbeta-deficient mice but not in ltbeta/tnf double-deficient mice. Additionally, stromal components of the spleen and LNs appear to be more severely disturbed in ltbeta/tnf double-deficient mice as compared with ltbeta-deficient mice. We conclude that LTbeta and TNF cooperate in the establishment of the correct microarchitecture of lymphoid organs.