It is intended to provide a remedy for mammalian mastitis, which exerts a remarkable effect when administered via a convenient administration route such as oral administration, nasal administration, injection or intramammary infusion, imposes little labor or burden to a care worker and little economical load to an owner and is scarcely risky in administrating to an animal, and a method of treating mastitis by using this remedy. The above problem can be solved by providing a remedy for mammalian mastitis which contains interferon as the active ingredient and a method of treating mammalian mastitis by using the remedy for mastitis as described above.