Three recent staining methods, the TPA-, TPL-, and TPT-method, were used to demonstrate cells of myothelial origin in mammary gland tumours in bitches and were compared with older techniques. The newer methods proved more suitable for demonstration of myofibrils in myothelial cells. With these techniques it is possible to reveal myofibril containing cells in adenomatous, papillary, myomatour, and myxoid tumour regions but not in chondroid parts of mixed tumours. Two of the tumours examined were classified as malignant myotheliomas because of their staining qualities, structure, ultrastructural appearance and signs of malignancy.