Although flavonoids manifest a diverse range of biological activities, including antitumor and antiviral effects, the molecular mechanisms underlying these activities await elucidation. We hypothesize that the flavonoid constituents of a proprietary grape seed extract (GSE) that contains procyandins exert significant antiviral and antitumor effects, by inducing production of the Th1-derived cytokine gamma interferon (IFN-gamma) by peripheral blood mononuclear cells) from healthy donors. Our results show that GSE significantly induced the transcription of IFN-gamma mRNA as demonstrated by reverse transcription-PCR but had no effect on the Th2-derived cytokine interleukin-6. The enhancing effect of GSE on IFN-gamma expression was further supported by a concomitant increase in the number of cells with intracytoplasmic IFN-gamma as well as the synthesis and secretion of IFN-gamma. Our results demonstrate that the potentially beneficial immunostimulatory effects of GSE may be mediated through the induction of IFN-gamma.