The simian immunodeficiency virus (SIV) isolate, SIVsmmPBj14, contains an immunoreceptor tyrosine-based activation motif (ITAM) within its nef gene product and triggers efficient lymphoproliferation in vitro. In experimentally inoculated macaque monkeys, this virus causes acutely lethal enteropathy, which is accompanied by high levels of pro-inflammatory cytokines, including tumor necrosis factor (TNF)-alpha. Since TNF-alpha has been shown to possess weak comitogenic activity for antigen- or mitogen-induced human T-cell proliferation, experiments were conducted to examine whether TNF-alpha might also contribute to SIVsmmPBj14-induced lymphoproliferation. Addition of a dimeric soluble human TNF receptor (sTNFR):Fc fusion protein to SIVsmmPBj14-infected simian peripheral blood mononuclear cells (PBMC) resulted in a partial (> 50%) inhibition of virally-induced lymphoproliferation, but had no effect on the strong T-cell activation signal provided by phytohemagglutinin and interleukin-2. Finally, the addition of exogenous human TNF-alpha to simian PBMC infected with a non-mitogenic variant of SIVsmmPBj14 failed to result in detectable lymphoproliferation, suggesting that TNF-alpha alone is not sufficient to cause the proliferation of SIV infected T-cells. Taken together, the data suggest that endogenous TNF-alpha enhances SIVsmmPBj14-induced lymphoproliferation in simian PBMC cultures.