Phage lysins are proteins used by phages to digest the bacterial peptidoglycan resulting in bacterial bursting. Lysins can be cloned, over-expressed and purified. Purified lysins have bactericidal activity against gram-positives when added from the outside. Very recently, lysins active against gram-negative – usually not affected because of the protective action of the outer membrane – were described. Lysins are therefore considered as very promising new antibacterial agents.