The Staphylococcus aureus bacteriophage phi11 endolysin has two peptidoglycan hydrolase domains (endopeptidase and amidase) and a SH3b cell wall-binding domain. In turbidity reduction assays, the purified protein can lyse untreated staphylococcal mastitis-causing pathogens, S. aureus and coagulase negative staphylococci (S. chronogenes, S. epidermis, S. hyicus, S. simulans, S. warneri, and S. xylocus), making it a strong antimicrobial protein and an effective candidate for treating multidrug-resistant staphylococci. Lytic activity is maintained at the pH (6.7) and the ‘free’ calcium concentration (3 mM) of milk. Truncated endolysin-derived proteins, containing just the endopeptidase domain, also lyse staphylococci, in the absence of the SH3b-binding domain.