Discloses a bovine granulocyte-colony stimulating factor (bG-CSF) polypeptide comprising a sequence of SEQ ID NO: 1 wherein the residue at position 133 is substituted for a non-naturally encoded amino acid or comprising a sequence of SEQ ID NO: 2 wherein the residue at position 134 tis substituted for a non-naturally encoded amino acid. The non-naturally encoded amino acid is para-acetylphenylalanine and bonded to a water soluble polymer comprising a poly(ethylene) glycol moiety. Also discloses the use of the modified peptides in the manufacture a medicament to treat infections mediated by bG-CSF or in the treatment of bovine mastitis.
