Lipopolysaccharides (LPS) are indispensable structural components of the Gram-negative bacterial outer membrane and are major determinants of virulence in pathogenic species. In the infected host LPS is better known as endotoxin where it acts as a potent stimulator of the inflammatory response. This article reviews the methods for the production and measurement of anti-LPS antibodies, and then describes the uses to which these methods have been employed. Antibodies to LPS (either monoclonal or polyclonal) may be used directly as immunotherapeutic agents for the treatment of Gram-negative sepsis or endotoxaemia, or as probes for the diagnosis and epidemiological investigation of Gram-negative bacterial infections. Antibodies are useful tools for investigation of the chemical structure of LPS, its expression on bacteria and to study the role of LPS in pathogenic mechanisms. The detection and quantitation of anti-LPS antibodies has formed the basis of classical and more recent serological studies of major bacterial infections.