The generalized lymphoproliferative disorder (gld) mouse strain is characterized by severe splenomegaly/lymphadenopathy, the production of autoimmune antibodies, and the appearance of CD4/CD8-negative T cells. An additional TNF deficiency of gld/gld mice attenuates the course of the disorder through a yet-unknown mechanism. In this study, we could demonstrate that the reduced splenomegaly and lymphadenopathy in B6.gld/gld. TNF(-/-) mice were correlated with a decreased peripheral T cell proliferation rate and a delayed polyclonal activation. A comparative analysis of naive T cells and memory/effector T cells showed an age-dependent difference in the T cell activation pattern in the spleen of B6.gld/gld and B6.gld/gld. TNF(-/-) mice. T cells from B6.gld/gld. TNF(-/-) spleens and lymph nodes showed significantly higher levels of CCR7 and CD62 ligand on their surface compared with B6.gld/gld mice when mice of the same age were compared. Additionally, we found an increased titer of the Th1 cytokine IFN-gamma in the serum of B6.gld/gld mice, whereas the concentration of IFN-gamma was markedly reduced in the serum of B6.gld/gld. TNF(-/-) mice. These findings support the hypothesis that increased T cell activation and proliferation in the presence of TNF contribute to the exacerbation of the gld syndrome. J. Leukoc. Biol. 85: 108-116; 2009.