Introduction: Mammary tumors are an important health problem in female dogs which constitute the most common neoplasm in this population. Transforming Growth Factor Beta (TGF Beta) family are believed to have both tumor suppressing and enhancing activities. In this study we purpose to investigate the expression of TGF Beta-1 in canine mammary tumors using Immunohistochemistry. Materials and Methods: 21 samples of canine mammary gland tumors included in this study, which had been confirmed histopathologically. Mouse ovary and canine normal mammary gland tissues used as positive and negative controls, respectively. Immunohistochemistry was done on formalin-fixed, paraffin-embdded samples with Avidin-Biotin Complex method (ABC) in which Horse Radish Peroxidase (HRP) and DAB used for developing positive signals. Images analysed using the software “Image J” v1.43 (U.S. National Institutes of Health, Bethesda, MD, U.S.A.) Results: Immunohistochemistry results revealed that 4 (19.04%) of tumor samples showed weak immunoreactivity, 10 (47.61%) with moderate and 7 (33.33%) with strong immunoreactivity for TGF beta-1. Conclusion: These results show that canine mammary gland tumors express TGF beta- 1 in different extent and it could be used as a therapeutic target in future.

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