A polysaccharide derivative that is the alpha-(2->4)-interlinked homopolymer of 5,7-diacetamido-8-O-acetyl-3,5,7,9-tetradeoxy-D-glycero-alpha-L-galacto-nonulosonic acid. It is the O-specific chain of the lipolysaccharide from Legionella pneumophila serogroup 1.