Lipopolysaccharides were isolated from Pseudomonas aeruginosa PAC1R and its lipopolysaccharide‐defective mutants. Core fractions were obtained from them following mild acid hydrolysis, by gel chromatography using Bio‐Gel P6. The core fraction from the parent strain contained glucose, rhamnose, galactosamine, heptose and alanine in the approximate molar ratios 3:1:1:1:1 while core fractions from the defective mutants lacked rhamnose and/or one, two, or three glucose units. Methylation analysis of the core fractions including identification of the partially methylated alditol acetates by combined gas‐liquid chromatography and mass spectroscopy allowed the formulation of possible structures. These were confirmed by ¹³C nuclear magnetic resonance spectroscopy. Fractions consisting of core to which was attached unpolymerised O‐antigen were isolated from the parent strain and one of the mutants and analysed in the same way to identify the attachment point of the O‐antigen.

The combined data leads to the formulation of the core oligosaccharide of PAC1R and its mutants as follows: