PURPOSE: To provide novel microbials belonging to Clostridium genus, and producing an anti-tumor polysaccharide C-45, and to prepare the polysaccharide C-45 by using said microbials.

CONSTITUTION: Bacteria producing insoluble polysaccharide C-45 (e.g. Clostridium sp. No.45) are separated from aceto-butanol-fermentation bacteria belonging to Clostridium genus, using PG medium consisting of potato, glucose, ammonium sulfate, calcium carbonate, and water. The separated bacteria are inoculated in a medium containing a carbon source, a nitrogen source, inorganic salts, and trace components, and cultured at 28-37°C and PH6-8. The C-45 substance accumulated as insoluble precipitate, is recovered and purified. The purification is carried out, e.g. by separating the precipitate from the liquid, dissolving to dimethylsulfoxide, fractionating by the precipitation using ethyl alcohol as precipitant, and dialyzing with water.