The pathogenic mechanisms of Gram-negative infection in cystic fibrosis are only just beginning to be explored at molecular level. Several virulence factors have been defined, one of the most important is the lipopolysaccharide molecule. In order to fully understand the mechanisms of bacterial infection and host recognition a full structure/activity study of lipopolysaccharide is needed. In the present paper, we define the complete structure of the O-specific polysaccharide from the lipopolysaccharide from Burkholderia anthina, an uncommon pathogen of cystic fibrosis patients.