Staphylococcus aureus infections can result in septic and toxic shock with depletion of immune cells and massive cytokine production. Recently, we showed that, in S. aureus-infected Jurkat T cells, alpha-toxin is the major mediator of caspase activation and apoptosis. Here, we investigated the mechanisms of cell death induced by alpha-toxin in peripheral blood mononuclear cells (MNC). We show that alpha-toxin is required and sufficient for S. aureus-induced cell death not only in transformed Jurkat T cells but also in MNC. Low alpha-toxin doses (3-30 ng ml-1) dose- and time-dependently induced apoptosis in both cell types, which was completely blocked by the caspase inhibitor zVAD-fmk. In Jurkat T cells and MNC, alpha-toxin induced the breakdown of the mitochondrial membrane potential and the intrinsic activation of caspase-3, -8 and -9. Interestingly, unlike in Jurkat T cells, apoptosis in MNC was additionally mediated by a caspase-9-independent component. MNC, but not Jurkat T cells, produced tumour necrosis factor (TNF)-alpha upon alpha-toxin stimulation. Blocking endogenous TNF-alpha with a TNF-alpha receptor antagonist partially decreased apoptosis in MNC. Our data therefore suggest that, whereas in Jurkat T cells apoptosis is solely mediated by the mitochondrial pathway, in MNC endogenous TNF-alpha and a death receptor-dependent pathway are also involved, which may contribute to depletion of immune cells during S. aureus infection.