In order to clarify the immunomodulating properties of ozone, we have investigated: a) the effects of stimulation on isolated peripheral human blood mononuclear cells (PBMC) from normal donors with either ozone or ozonated serum; b) the range (in terms of O3 concentrations) of the therapeutic window; c) the stimulatory and toxic effects and d) the pattern, of both proinflammatory and immunosuppressive cytokine production up to 86 hours after exposure to O3. Results show that ozone can act as a weak inducer of cytokines producing IL-6, IL-4, TNF-a, IFN-g, IL-2 and IL-10 and, most importantly, there is a significant relationship between cytokine production and ozone concentration. Analysis of the proliferation index shows that progressively increasing O3 concentrations inhibit IP and therefore appear cytotoxic.