AbstractModulatory effect on the murine self defense system by a newly discovered acidic polysaccharide (ANK-102) produced byP. tuberosacells in liquid culture was examined. Pretreatment with ANK-102 deteriorated the murjne survival against lethal infection ofListeria monocytogenes, an intracellular grampositive bacterium eliminated mainly by macrophages through T-cell mediated immune response. Pretreatment with ANK-102 resulted in the accumulation of Mac 1 and Mac 2 positive cells in the peritoneal cavity of the infected animals and the reduction of Thy 1.2 expression on the surface of the thymocytes. A new type of immunosuppressive polysaccharide ANK-102 was introduced.