Highly purified lipopolysaccharide (LPS) preparations obtained from seven Actinobacillus pleuropneumoniae strains representative of seven different serotypes were used to determine the structure and monosaccharide composition of the polysaccharide components of each lipopolysaccharide. An indication of the structure of each LPS was obtained by procedures that included sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by silver staining and gel chromatographic fractionation of acetic acid-hydrolyzed LPS. The polysaccharide components of the LPSs were analyzed by gas-liquid chromatography. The LPSs of the strains of serotypes 2, 4, and 7 were of the smooth type, and those of the strains of serotypes 3 and 6 were of the rough type; the LPSs of the strains of serotypes 1 and 5 could be considered semirough. Rhamnose was present only in the O polysaccharide of the smooth-type and semirough-type LPSs, whereas galactose was present only in the O polysaccharide of the smooth-type LPS and in the core oligosaccharides of the rough-type and semirough-type LPSs. Glucoheptose and mannoheptose were present in the core oligosaccharides of all the LPSs except for the strain of serotype 3, in which only mannoheptose was detected. N-Acetylglucosamine was detected only in the O polysaccharides of the strains of serotypes 1 and 5.