Bactericidal/permeability-increasing protein is a member of the BPI/LBP/Plunc superfamily and BPI/LBP family. It is a cationic protein which can be detected in the azurophilic granule and on the surface of polymorphonuclear leukocytes. Bactericidal/permeability-increasing protein also is a lipopolysaccharide binding protein. It is associated with human neutrophil granules and has bactericidal activity on gram-negative organisms. Bactericidal/permeability-increasing protein contains two domains that adopt the same structural fold, even though they have little sequence similarity. It binds to and neutralises lipopolysaccharides from the outer membrane of Gram-negative bacteria. The cytotoxic action of bactericidal/permeability-increasing protein is limited to many species of Gram-negative bacteria; this specificity may be explained by a strong affinity of the very basic N-terminal half for the negatively charged lipopolysaccharides that are unique to the Gram-negative bacterial outer envelope.