S-form lipopolysaccharide was isolated by phenol/water extraction from a strain of Acinetobacter calcoaceticus (DNA group 1 ). The structure of the O-antigenic polysaccharide was determined by compositional analysis and NMR spectroscopy of the de-O-acylated lipopolysaccharide. The isolated polysaccharide obtained after hydrolysis of lipopolysaccharide in 0.01 M trifluoroacetic acid has the following structure: [STRUCTURE IN TEXT] in which Pyr is pyruvate. The O-acetyl substitution of D-Gal was non-stoichiometric. The O-antigen was specifically recognised in western blots by polyclonal rabbit antisera.