The endotoxins are small hydrophobic molecules, lipopolysaccharides (LPS), associated with the outer membranes of certain microorganisms and more particularly the Gram-negative bacteria (Brucella, Neisseria, Vibrio ...). These small thermostable molecules are not secreted but are released only when the cells are destroyed. In large quantities, endotoxins can cause haemorrhagic shock and acute diarrhea, lower levels may cause fever, lower resistance to bacterial infections, leukopenia followed by leukocytosis and other biological effects, depending of organism and its physiological conditions.