Mercury induces autoimmune disease and increases IL-4 production and IgE levels in certain rodent strains. The object of this study was to determine if mercury was capable of inducing Th2 cytokine production in human leucocytes. Human peripheral blood mononuclear cells (PBMC) were incubated with PMA/ionomycin or Con A in the presence or absence of methyl mercury (CH₃Hg) or mercuric chloride (HgCl₂). IL-4 and gamma-IFN were measured by ELISA. RESULTS: IL-4 production significantly increased at low concentrations of CH₃Hg (0.5 uM, p < 0.01), while gamma-IFN production was suppressed starting at CH₃Hg 2 uM (p = 0.004). Inorganic mercury (HgCl₂) increased IL-4 only at concentrations 10-20 times higher than CH₃Hg. These findings suggest a mechanism by which mercury could trigger or potentiate TH2 cytokine production in humans.