The present invention provides to a polyethylene glycol-modified feline-derived protein which is obtained by chemically modifying a feline-derived protein with polyethylene glycol. The feline-derived protein is produced by a method comprising any or a combination of extracting the protein from somatic cells of a transgenic bird and/or an egg laid thereby, purifying and activating the same. The transgenic bird has a foreign gene containing a sequence encoding a feline-derived protein.
