Main attention in the paper is paid to the study of lipid A, a component possessing endotoxic activity. Lipids A containing glucosamine disaccharide (representatives of Enterobacteriaceae family), and variants of lipid A differing from the toxic one either in the structure of carbohydrate core or in the spectrum of fatty acids are considered. They are either phototrophic, nodulating (Bradyrhyzobium species) or soil species (Nitrobacter and Thiobacillus) bacteria. Lipid A from lipopolysaccharides of over 25 species of bacteria (Rhodopseudomonas viridans, R. palustris, Pseudomonas diminuta, Phenylobacterium immobile, Brucella melitensis, B. abortus, Thiobacillus ferrooxidans, etc.) contains 2.3-diamino-2.3-dideoxyglucose (lipid ADAG); glucosaminouronic acid was found in Rhizobium trifolii and galacturonic acid in R. leguminosarum bvs. phaseoli, trifolii and viceae. Mixed lipids (lipid A and lipid ADAG) were found in Campylobacter jejuni. Considerable variations were registered in the nature of fatty acids. Thus, 27-oxy-octacosanoic acid (27-OH-28 : 0) was found in lipid A of the studied species of Rhizobiaceae except for Azorhizobium caulinodans. No correlations between the composition of the carbohydrate core and presence of this acid were established. Implementation of the synthesis of a complete as well as of partial lipid A structures has confirmed authenticity of the described structures. Five different epitopes identified by antibodies are present in the hydrophilic part of lipid A. The structure and biological role of the outer and inner cores are considered separately, main attention being paid to identification of the role of the KDO-containing zone. Since O-specific polysaccharide is the most known lipopolysaccharide component from the viewpoint of the structure and biological activity, this material is given in a general form.