In vitro studies have demonstrated positive effects of bioactive compounds on several functions of the immune system. In the present study, 25 of such compounds were tested for their immune modulating properties on influenza virus specific human B- and T-cell responses in vitro. One of these compounds, N-acetyl-L-cysteine was shown to increase influenza virus specific lymphocyte proliferation and interferon(IFN)-γ production at a concentration of 1.0 mmol/1. Furthermore, N-acetyl-L-cysteine was found to enhance a specific activity of two influenza specific CD8+ cytotoxic T-lymphocyte clones directed towards HLA-A*0201 and HLA-B*2705 restricted epitopes. A second compound, chlorogenic acid, was shown to enhance antigen specific proliferation of lymphocytes in three out of four donors, at concentrations of 10-50 μmol/1. Neither of the two compounds exhibited a positive effect on the production of influenza virus specific antibodies by human peripheral blood mononuclear cells in vitro.