Toxicity of chemotherapeutic drugs towards normal cells is a serious side effect of cancer treatment. Thus, finding of molecules with low toxicity for normal cells is crucial. Several natural compounds, such as flavonoid quercertin, are receiving a growing attention as "chemopreventers". Quercetin kills tumour-derived cell lines, but little is known about its effects on normal cells. Here we show that although quercetin exerts a higher apoptotic potential on leukemic cell lines than on peripheral blood mononuclear cells (PBMCs) and does not sensitize PBMCs to CD95-induced apoptosis, it is able to inhibit normal immune functions such as T cell proliferation and activation. Quercetin sensitivity is independent on cell cycle progression since it was not abrogated in serum-starved U937 cells, nor proliferating PBMCs underwent apoptosis after quercetin treatment. However, quercetin prevented PHA-induced PBMC proliferation and SEB-induced upregulation of activation markers. Our data suggest that quercetin, while incapable of inducing apoptosis in normal cells under several conditions, could interfere with effector T cell function.