In a process for the production of a staphylococcal antigen product, intact cells of a strain of Staphylococcus aureus identifiable by the property of forming loosely-cohesive, diffuse, semi-opaque colonies when grown in a semi-solid or soft agar culture medium containing plasma, serum or protein components thereof, are reacted with an aqueous acid at a pH of not more than 4 and the resulting aqueous solution containing an immunogenic polysaccharide is separated from insoluble solid material, the aqueous solution, upon adjustment to a pH acceptable for injection, being capable of stimulating the formation of a protective anti-body against Staphylococcus aureus. Specified Staphyloccus aureus strains are UC76, Smith, S193N4, H & D, Castellani, MCD138, SK4473, K6, K93 and K93M. The intact cells may be live or "killed" by e.g. heat-treatment, or treatment with phenol or b -propiolactone. The acid reactant may be e.g. aqueous acetic, formic, propionic, mono-chloroacetic, tri-chloroacetic or hydrochloric acids. The aqueous solution of polysaccharide, after separation from solids, may be used as a vaccine, the pH being adjusted to one suitable for parenteral injection, e.g. pH 6-8. The vaccine may be made isotonic by addition of sodium chloride and sterilized by heat, ultra-violet irradiation, or by addition of a preservative such as phenol, thimerosol or benzethonium chloride. Alternatively, the polysaccharide may be isolated from its aqueous solution by e.g. dialysis and freeze-drying, and may be purified by e.g. chromatography, precipitation of impurities, or by formation of a quaternary ammonium complex.  The polysaccharide contains carboxyl groups and may be obtained as free acid, or in the form of sodium, potassium, calcium, magnesium, ammonium or amine salts. In examples, cultures of the UC76 strain of Staphylococcus aureus are treated with 0.1N acetic acid at 100 DEG  C., 1N and 0.1N acetic acid at 121 DEG C. and tri-chloroacetic acid at room temperature. The extracted polysaccharide resembles in properties polysaccharide II disclosed in the parent Specification.ALSO:In a process for the production of a staphylococcal antigen product, intact cells of a strain of staphylococcus aureus identifiable by the property of forming loosely-cohesive, diffuse, semi-opaque colonies when grown in a semi-solid or soft agar culture medium containing plasma, serum or protein components thereof, are reacted with an aqueous acid at a pH of not more than 4 and the resulting aqueous solution containing an immunogenic polysaccharide is separated from insoluble solid material, the aqueous solution, upon adjustment to a pH acceptable for injection, being capable of stimulating the formation of a protective antibody against staphylococcus aureus.  Specified staphylococcus aureus strains are UC76, Smith, S 193N4, H & D, Castellani, MCD138, SK4473, K6, K93 and K93M.  The intact cells may be live or "killed" by e.g. heat-treatment, or treatment with phenol or b -propiolactone.  The acid reactant may be, e.g., aqueous acetic, formic, propionic, monochloroacetic, tri-chloroacetic or hydrochloric acids.  The aqueous solution of polysaccharide, after separation from solids, may be used as a vaccine, the pH being adjusted to one suitable for parenteral injection, e.g. pH 6-8.  The vaccine may be made isotonic by addition of sodium chloride and sterilized by heat, ultra-violet irradiation, or by addition of a preservative such as phenol, thimerosol or benzethonium chloride.  Alternatively, the polysaccharide may be isolated from its aqueous solution by, e.g. dialysis and freeze-drying, and may be purified by, e.g. chromatography, precipitation of impurities, or by formation of a quaternary ammonium complex.  The polysaccharide contains carboxyl groups and may be obtained as free acid, or in the form of sodium, potassium, calcium, magnesium, ammonium or amine salts. In examples, cultures of the UC76 strain of staphylococcus aureus are treated with 0,1N acetic acid at 100 DEG  C., IN and 0,1N acetic acid at 121 DEG C., and tri-chloroacetic acid at room temperature.  The extracted polysaccharide resembles in properties polysaccharide II disclosed in the parent Specification.