The surface polysaccharides of the two most recently proposed O-serotype strains of Serratia marcescens, O25 and O26, were characterised in terms of their chemical structure and immunological reactions. No polymer was isolated from O25, which was shown to lack both capsular K-antigen and smooth, O-antigenic lipopolysaccharide. A neutral polysaccharide was isolated from O26 and shown to be a polymer of rhamnose and N-acetylgalactosamine of the type previously found in the O9 and O15 reference strains. Serological cross-reactions among all three strains were demonstrated by using both whole-cell enzyme-linked immunosorbent assay and immunoblotting of lipopolysaccharide resolved by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. No acidic polysaccharide was found in O26 and this was consistent with the absence of an immunogenic capsule. Thus, neither strain qualifies for inclusion as a new serotype in either an O-typing or a K-typing scheme.