The presence of metallothioneins (MTs) were demonstrated immunohistochemically using a monoclonal antibody (E9) against a conserved epitope of I and II isoforms in canine mammary tumours. In a semiquantitative analysis MT expression in the tumour cells was observed in 54/54 cases of benign and 32/40 malignant mammary neoplasms. A statistically significant difference at the level of P<0.01 was observed for MT expression between benign and malign mammary tumours in terms of immunoreactivity score. It is concluded that immunohistochemically demonstrated MT expression is significantly associated with benign canine mammary tumours.