trans-Resveratrol, a phytoalexin found in grapes, wine, and other plant products, has been shown to have anti-inflammatory, antioxidant, and antitumor activities. Many of these beneficial effects of resveratrol require participation of the cells of the immune system; however, the effect of resveratrol on the development of immunological responses remains unknown. We have investigated the effect of resveratrol on mitogen/antigen-induced proliferation of splenic lymphocytes, induction of cytotoxic T lymphocytes (CTLs) and lymphokine activated killer (LAK) cells, and the production of the cytokines interferon (IFN)-γ, interleukin (IL)-2, tumor necrosis factor (TNF)-α, and IL-12. We found that mitogen-, IL-2-, or alloantigen-induced proliferation of splenic lymphocytes and the development of antigen-specific CTLs were suppressed significantly at 25–50 μM resveratrol. The generation of LAK cells at similar concentrations was less sensitive to the suppressive effect of resveratrol. The suppression of cell proliferation and CTL generation by resveratrol was not only reversible, but in some cases the response (mitogen/IL-2-induced proliferation and CTL generation) was actually enhanced following pretreatment of cells with resveratrol. Resveratrol also inhibited the production of IFN-γ and IL-2 by splenic lymphocytes, and the production of TNF-α and IL-12 by peritoneal macrophages. The inhibition of cytokine production by resveratrol was irreversible. Further, resveratrol blocked the activation of the transcription factor NF-κB without affecting basal NF-κB activity. The latter result suggests that resveratrol inhibits cell proliferation, cell-mediated cytotoxicity, and cytokine production, at least in part through the inhibition of NF-κB activation.