By using trichloracetate method, diethylene glycol method, and trypsin digestion method, the author has separated a polysaccharide from B. Proteus X19 and refined it, and then having compared the respective quantity and purity derived from each method, the following results have been obtained. 1) The quantity derived in each method has been found in the order of trichloracetate method > trypsin method > diethylene glycol method. 2) The comparison of the purity so determined has been found diethylene glycol method > trichloracetate method > trypsin method, in the order mentioned. 3) The quantity of polysaccharide after extraction (digestion) is reduced to 1/3.6-1/28.5 on account of the refining by the Seitz-Werke filter. When dried and pulverized, it has been further reduced to 1/10-1/20. 4) It is possible to preserve this polysaccharide for a long period of time. 5) Having examined the chemical nature of polysaccharide abstracted by trichloracetate method, and after hydrolysis with paper-chromatography, three saccharide-like spots have been obtained. 6) This polysaccharide has been recognized to be a monoantigen by agar -method. 7) The introduction of P(32) into Boivin type antigen has been ascertained.