We studied the effect of sulfurous water thermal therapy on the phenotype and the proliferative response of peripheral lymphoid cells from ten subjects affected by chronic upper respiratory disease and from six suffering from articular and periarticular disorders. Sulfurous water (S-H2O) therapy did not modify the phenotype and function of peripheral blood mononuclear cells (PBMC) nor did it modify systemic immunologic reactivity. A different result was obtained by analyzing the response to mitogens of peripheral blood mononuclear cells in cell cultures containing graduated amounts of S-H2O. These "in vitro" studies have shown an important dose-dependent inhibitory effect of S-H2O on mitogen induced T lymphocyte proliferation and on IL2 production. H2S present in S-H2O seems to be the primary component responsible for inhibition. Our results are consistent with a local immunosuppressive role of S-H2O, which may explain part of the observed therapeutic effect of inhalation therapy on upper respiratory allergic disorders.