Canine tumors, especially canine breast cancer, share common properties, including metastatic pattern, histopathological characteristics, and biological features with human cancers. Therefore, canine mammary gland tumor is deemed as an appropriate model for human breast cancer. We investigated expression of two important tumor antigens, including manganese superoxide dismutase (Mn-SOD) and triose-phosphate isomerase (TPI) in canine mammary gland tumors using immunohistochemistry analysis. Expression of these proteins were evaluated in tumor (29 malignant and 16 benign samples) and normal (n=9) tissues. Protein expression was scored according to a standard method and correlation of protein expression was analyzed with clinicopathological features, including animal age, tumor size, histological type, histological grade, and lymph node metastasis. Overexpressions of Mn-SOD and TPI were detected in 9 (31 %) and 14 (48.3 %) malignant and 3 (18.7 %) and 5 (31.3 %) benign samples, respectively, where overexpressed samples were more frequent in malignant cases than in benign cases (Mn-SOD: P=0.049, TPI: P=039). Moreover, expression level of Mn-SOD significantly correlated with tumor grade (P=0.030). No significant correlation was detected between Mn-SOD or TPI immunoreactivity and other clinicopathological parameters.