PROBLEM TO BE SOLVED: To obtain a new dog interleukin 18 having an activity for inducing a factor acting on a canine leukocyte to intensify the expression of an antiviral activity factor and class II MHC existing on a canine tumor cell or the like, and useful as a treating agent for canine immunological diseases or the like. SOLUTION: A new canine interleukin 18 acting on a canine leukocyte to give at least one among an activity for inducing a factor for intensifying the expression of an antiviral activity factor and class II MHC existing on a canine tumor cell, an activity for stimulating the growth of canine lymphocytes, an activity for intensifying the expression of Fas ligand existing on a canine lymphocyte and a canine tumor cell, an activity for damaging and killing a canine tumor cell, an activity for reducing the size of tumors occurred on a dog body, an activity for activating a canine leukocyte to suppress canine allergy, and the like. The substance is useful as a treating agent for canine immunological diseases or the like. It is obtained by expressing a recombinant vector including a DNA encoding the precursor protein of interleukin 18 and a DNA encoding an interleukin-1 &beta； converting enzyme.
