The structure of the O-antigen polysaccharide of A. pleuropneumoniae serotype 7 was investigated by methylation analysis, partial acid hydrolysis, periodate oxidation, and H-1- and C-13-n.m.r. spectroscopy. The polysaccharide repeating-unit consists of a branched tetrasaccharide having the following structure. [--> 4)-alpha-L-Rhap-(1--> 3)-beta-D-Galp-(1--> 4)-beta-D-GalpNAc-(-->]n [GRAPHICS]