The lipopolysaccharide(LPS) of Legionella spp. is an immuno-dominant antigen and the basis for Legionella pneumophila serogroup classification. The LPS shows a peculiar structure composed of a very hydrophobic lipid A acylated by long chain fatty acids and an O-antigen-specific chain consisting of homopolymeric legionaminic acid. In this chapter we describe a method for the isolation of LPS from L. pneumophila. In the first part we describe the chemical purification, in the second part we outline the application of monoclonal antibody (mAb) in Western blot and immuno-localization by indirect immunofluorescence. This report does not describe physico-chemical methods that analyze the structure of lipopolysaccharide entities.