Interferon gamma (IFN) represents an essential cytokine involved in murine cytomegalovirus (MCMV) clearance from the salivary gland and the control of horizontal transmission. Because IFN cannot be responsible for all cytokine effects during recovery from MCMV infection we have now tested the potential participation of tumour necrosis factor alpha (TNF) in the antiviral defence. Neutralization of endogenous TNF abolished the antiviral activity of CD4 T cells in immunocompetent as well as in CD8 subset-deficient mice. These data suggest that the antiviral effect of the CD4 subset requires the presence of at least two cytokines, namely IFN and TNF. Depletion of endogenous TNF in adoptive cell transfer recipients diminished the antiviral function of CD8 T lymphocytes suggesting that TNF also participates in CD8 T cell effector functions. Furthermore, endogenous cytokines were found to be required for survival after infection with lethal doses of MCMV, whereas immunotherapy with recombinant TNF and IFN could not limit virus replication in vivo. The results suggest that, similar to IFN, TNF is an integral part of the protective mechanisms involved in cytomegalovirus clearance.