Mammary tumours are the most common tumours in the female dog. The tumours have a complex histology and exist in epithelial, mixed and mesenchymal forms. To study the biology of canine mammary tumours, five cell lines have been established and characterized. The results indicate that canine mammary tumours might be derived from mammary stem cells and that the tumour growth is independent of oestrogens. The established canine mammary tumour cell lines will be valuable tools in further studies of the histogenesis and pathogenesis of these tumours.