The study is aimed to determine the role of quercetin (3,3'4',5,7-pentahydroxy flavone), alone and in combination with human interferon-beta (IFN-beta), in modulating the immune response(s) of peripheral blood mononuclear cells (PBMC) isolated from multiple sclerosis (MS) patients and from normal healthy subjects. PBMC proliferation in the presence or absence of these drugs was determined and the production of proinflammatory cytokines (IL-1beta, TNF-alpha), and the ratio of cell migration mediator MMP-9, and its inhibitor, TIMP-1 were assessed in the culture supernatants. Quercetin reduced, in a dose-dependent manner, the proliferation of PBMC and modulated the level of IL-1beta and TNF-alpha released by PBMC in the culture supernatants. Quercetin reduced the MMP-9/TIMP-1 ratio via lowering MMP-9 production. Quercetin, when combined with IFN-beta, had additive effects in modulating TNF-alpha and MMP-9. These immunomodulatory responses to quercetin were similar between MS patients and healthy control (HC) subjects.