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From E. coli 0100:K?(B):H2 a lipopolysaccharide and an acidic polysaccharide were extracted with saline. The lipopolysaccharide was purified by repeated ultracentrifugation and the acidic polysaccharide by fractional precipitation with cetavlon. Both substances contained rhamnose, galactose, glucosamine, glycerol, and phosphate. The lipopolysaccharide was degraded in 1% acetic acid at 100° and the degraded polysaccharide was fractionated on Sephadex G-50. Thus a fraction was obtained which represents the specific polysaccharide moiety of the lipopolysaccharide. comparison of this fraction with the acidic polysaccharide (chemical analysis, periodate oxidation, alkaline hydrolysis, partial acid hydrolysis) showed that both substances have not only the same composition (rhamnose, galactose, glucosamine, glycerol and phosphate in the molar ratios of 2:1:1:1:1) but also the same structure. Both substances consist of a main chain (composed of rhamnose, galactose and glucosamine) to which glycerol is bound via a phosphodiester bridge from one of its primary hydroxyl groups to one of the rhamnose residues. Some alternatives for the structure are presented. In serological tests (passive haemagglutination, bacterial agglutination, immune precipitation and inhibition of these) it was shown that acidic polysaccharide and lipopolysaccharide have identical reactivities in anti-O and anti-OK antisera. This confirms the structural identity of the polysaccharides. It is concluded that E. coli 0100:K?(B):H2 contains no polysaccharide which represents a K antigen. The O antigen of this strain contains glycerol phosphate as (part of) a determinant group. Precipitating antibodies, directed against this group are present in anti-OK antiserum and not in anti-O antiserum. In this context the phenomenon of O inagglutinability of E. coli atrains, which carry K antigens, is discussed. Possible implications in the definition of K antigens of the B type in human enteropathogenic E. coli strains are presented. [ABSTRACT FROM AUTHOR]