The present invention relates to a polypeptide with endolysin activity comprising an amino acid sequence according to SEQ ID No. 1 and fragments or derivatives thereof, or fusion proteins derived thereof. Moreover, the present invention relates to nucleic acid molecules encoding said polypeptide or fusion protein, vectors comprising said nucleic acid molecules and host cells comprising either said nucleic acid molecules or said vectors. In addition, the present invention relates to said polypeptide, fragment, derivative or fusion protein for use as a medicament, in particular for the treatment or prevention of Gram-negative bacterial infections, as diagnostic means, as cosmetic substance or as sanitizing agent. The present invention also relates to the use of said polypeptide, fragment, derivative or fusion protein for the treatment or prevention of Gram-negative bacterial contamination of foodstuff, of food processing equipment, of food processing plants, of surfaces coming into contact with foodstuff, of medical devices, of surfaces in hospitals and surgeries. Furthermore, the present invention relates to a pharmaceutical composition comprising said polypeptide, fragment, derivative or fusion protein.