We cloned the canine interleukin-12 (IL-12) subunit cDNA. Canine IL-12 exhibited sequence homology to the known sequences of human, mouse, and bovine genes at nucleotide and amino acid levels. Cotransfection of the p35 and p40 subunits of canine IL-12 cDNA clones into COS-1 cells resulted in the secretion of IL-12, which supported proliferation of the stimulated canine lymphocytes, promoted induction of canine interferon-gamma (IFN-gamma) from canine lymphocytes, and showed antitumor effect in vitro. The cloned canine IL-12 will be useful for canine therapeutic applications.