The complement plays the most important role in eliminating bacterial invasion of the host, by facilitating phagocytosis of potential pathogens and by participating in the direct essential role in protecting gram-negative bacteria against bactericidal activity of serum. Sialic acids which are important constitutes of animal tissue glycoconjugates are also present in antigens of some bacterial strains. The susceptibility of gram-negative strains with sialic acid--containing lipopolysaccharides to bactericidal action sera was examined.