Trans-resveratrol, a natural stilbene present in wine and grapes, has been studied mainly for its antiinflammatory and anticancer activities. In this study the activity of resveratrol on proliferative immunological parameters (differentiation, apoptosis, phagocytosis and intracellular killing) was studied using a U937 human promonocytic cell line in comparison with another polyphenol, quercetin. After incubation of the pathogen, Candida albicans, intracellular killing by macrophage-like cells was decreased by quercetin and resveratrol 10 microM but was enhanced by resveratrol 1 microM after 20 h of treatment. Phagocytosis rate, expressed as phagocytosis frequency, (i.e., percentage number of phagocytosing cells/total cells) at 20 h was highest with resveratrol 10 microM and was higher with quercetin 10 microM than with resveratrol 1 microM. The phagocytosis index exhibited the same trend. While both polyphenols demonstrated cytostatic activity on U937 growth, a prointraphagocytic effect for resveratrol 10 microM-treated cells at 10 min, resveratrol 1 microM-treated cells at 20 h and resveratrol 10 microM-treated cells at 48 h was observed. Morphological examination with optic microscopy demonstrated both apoptotic and differentiating cells, even after 10 min treatment. Resveratrol-induced apoptosis (following 4 h treatment) was confirmed by flow cytometry at concentrations as low as 1 microM and 100 nM in the assay for detection of membrane phosphatidylserine. Resveratrol- or quercetin-treated, but unstimulated cells, did not produce tumor necrosis factor-alpha protein. As phosphatidylserine externalization triggers specific recognition by monocytes and macrophages, removal of intact apoptotic cells is important a) in cell population selection and differentiation for antiblastic therapy, and b) in preventing the release of toxic inflammatory substances such as reactive oxygen substances and proteolytic enzymes by dying cells. This observation suggests that wine polyphenols, at the same concentrations as those found in plasma after moderate wine consumption, are important cofactors in antiinfective, antiinflammatory and anticancer nonspecific immune reactions.