A soil bacterium, Klebsiella oxytoca TNM3, was found to produce a polysaccharide named AZ9 with immunosuppressive activities. The culture of K. oxytoca in a jar fermentor resulted in an extreme increase in the viscosity of the culture, broth (>10,000 mPa (.) s) due to accumulation of the polysaccharide with a high molecular weight. The molecular weight of AZ9 was successfully controlled by supplementing ethylenediamine tetraacetic acid (EDTA) and Fe2+ ion to the culture. Under optimized culture conditions, 39 g/l AZ9 with an average molecular weight of 200 kDa was produced, which was effective in suppressing IgE antibody production in mice.