IL-10 is considered a potent antiinflammatory cytokine that strongly inhibits the production of proinflammatory cytokines. Recent studies have suggested that IL-10 also has immunostimulatory properties on CD4+, CD8+ T cells, and/or NK cells, resulting in increased IFN-γ production. To determine the effect of IL-10 on IFN-γ production and related inflammatory responses in humans, 16 healthy subjects received a bolus iv injection of LPS (4 ng/kg) in combination with either placebo or recombinant human IL-10 (25 μg/kg), administered just before or 1 h after LPS. IL-10 treatment, particularly when administered after LPS, enhanced LPS-induced IFN-γ release, as well as the release of the IFN-γ-dependent chemokines IFN-γ-inducible protein-10 and monokine induced by IFN-γ, while inhibiting or not influencing the production of IFN-γ-inducing cytokines. In addition, IL-10 treatment enhanced activation of CTLs and NK cells after LPS injection, as reflected by increased levels of soluble granzymes. These data indicate that high-dose IL-10 treatment in patients with inflammatory disorders can be associated with undesired proinflammatory effects.