<;P>;PROBLEM TO BE SOLVED: To provide an injection agent for treating a mammalian (especially bovine) mastitis, using nisin as an active ingredient. <;P>;SOLUTION: The injection agent contains the nisin preferably powdered by using a water-soluble binder, in a shape dispersed in an oily base. The agent preferably contains thioctic acid as a stabilizer of the nisin to provide the excellent delivery thereof in the breast. <;P>;COPYRIGHT: (C)2011,JPO&INPIT