Listeria monocyogenes is a faculative intracellular Gram-positive food borne bacterium, increasingly recognized as being responsible for severe infections in both humans and animals. Ingestion of contaminated food causes an infection called Listeriosis, which affects especially immunocomprimised patients, new borns and pregnant women. It is characterized by severe syndromes such as encephalitis, meningoencephalitis, septicemia and abortion. Listeriolysin O (LLO) is a single polypeptide protein secreted by the Gram-positive bacterium Listeria monocytogenes. LLO belongs to the group of cholesterol-binding sulfhydrylactivated toxins, the lytic activity of which is enhanced by reducing agents and is suppressed by exposure to oxygen or cholesterol. LLO hemolytic activity is maximum at pH 5.5 and rapidly decreases with the increase of the pH.