Endotoxins of Gram-negative microbes fulfill as components of the outer membrane a vital function for bacterial viability and, if set free, induce in mammalians potent pathophysiological effects. Chemically, they are lipopolysaccharides (LPS) consisting of an O-specific chain, a core oligosaccharide, and a lipid component, termed lipid A. The latter determines the endotoxic activities and, together with the core constituent Kdo, essential functions for bacteria. The primary structure of lipid A of various bacterial origin has been elucidated and lipid A of Escherichia coli has been chemically synthesized. The biological analysis of synthetic lipid A partial structures proved that the expression of endotoxic activity depends on a unique primary structure and a peculiar endotoxic conformation. The biological lipid A effects are mediated by macrophage-derived bioactive peptides such as tumor necrosis factor alpha (TNF). Macrophages possess LPS receptors, and the lipid A regions involved in specific binding and cell activation have been characterized. Synthetic lipid A partial structures compete the specific binding of LPS or lipid A and antagonistically inhibit the production of LPS-induced TNF. LPS toxicity, in general, and the ability of LPS to induce TNF are also suppressed by a recently developed monoclonal antibody (IgG2a), which is directed against an epitope located in the core oligosaccharide. At present we determine molecular and submolecular details of the specificity of the interaction of lipid A with responsive host cells with the ultimate aim to provide pharmacological or immunological therapeutics that reduce or abolish the fatal inflammatory consequences of endotoxicosis.