An extracellular polysaccharide (referred to as Ps-III), which showed a positive immune activity to anti-Ps-III-sera obtained from a rabbit immunized with Escherichia coli 29M organisms, was isolated from the culture broth of E. coli 29M. By methylation, Smith degradation, partial acid hydrolysis and methanolysis of Ps-III, its repeating structure was postulated as given below : [numerical formula]