Sialic acids which are important constituents of animal tissue glycoconjugates are also present in antigens of some bacterial strains. Capsular polysaccharides with sialic acid have been extensively studied whereas little is known on lipopolysaccharides which contain sialic acid. The paper presents review of the data concerning structure of bacterial endotoxic lipopolysaccharides. Methodological peculiarities were described. Specially emphasized were endotoxins of Escherichia coli O24, O56, O104, Salmonella toucra O48, Citrobacter freundii O37 and Hafnia alvei strain PCM2386.