The present invention relates to a polypeptide with endolysin activity and amino acid sequences according to SEQ ID No. 1 and fragments or derivatives thereof. Moreover, the present invention relates to nucleic acid molecules encoding said polypeptide, 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 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 for the treatment or prevention of bacterial contamination, particularly of Gram-negative 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 doctor's offices. Furthermore, the present invention relates to a pharmaceutical composition comprising said polypeptide.