The chemical structure of the O-specific polysaccharide chain of Proteus penneri 62 lipopolysaccharide (LPS) containing N-acetylisomuramic acid was established using acid hydrolysis, solvolysis with anhydrous hydrogen fluoride and 1H and 13C NMR spectroscopy. Cross reactivity of the anti-O-serum P. penneri 62 with a number of other strains of the same species isolated in the USA, Canada, Germany and Poland is discussed.