Lipopolysaccharides were isolated from one strain of Leptospirillum ferrooxidans and three strains of Thiobacillus ferrooxidans. All strains contained 2-keto-3-deoxyoctonate (KDO), heptose(s) and glucosamine besides neutral sugars, but were devoid of phosphorus. Lipopolysaccharides from iron-grown cells and from sulfur-grown cells had a similar chemical composition. Thus, the chemical properties of the lipopolysaccharide was unaffected by the energy source utilized.Chemical analysis of lipopolysaccharide from Thiobacillus ferrooxidans IFO 14262 revealed the presence of L-rhamnose, D-glucose, L-glycero-D- nianno-heptose, KDO, D-glucosamine, and a lipophilic sugar identified as L-acofriose (3-O-methyl-L-rhamnose). 3-Hydroxymyristic acid was the main fatty acid. By hydrolysis in weak acid, the lipopolysaccharide has been separated into the polysaccharide part ("degraded polysaccharide") and lipid A. Presumably the lipid A contains a glucosamine backbone.