Abstract: For the first time, an O-specific polysaccharide antigen that is a Shigella Sonnei, phase I, exopolysaccharide has been produced and characterized, said exopolysaccharide being an authentic natural compound in the form of a bacterial capsular polysaccharide. The exopolysaccharide contains a non-toxic lipid component, namely non-hydroxylated fatty acids, and exhibits low pyrogenicity and high immunogenicity. Without using lipopolysaccharides as the source of production, an exopolysaccharide with a high degree of purity is produced from a liquid phase culture of S. sonnei bacteria by means of a workable industrial method with a high yield.