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Mammary tumours are the second most diagnosed tumours after skin tumours in bitches; however, the aetiology is still unknown and this calls for continuous investigations. The present study aimed to surgically excise and evaluate a solitary mass on the right first axillary mammary gland of a 9-year-old Caucasian bitch presented for clinical attention. The surgical intervention was according to standard procedure under general anaesthetic protocols. Histopathological and immunohistochemical evaluation of the tumour followed standard procedures after fixing in 10% neutral buffered formalin. The immunohistochemical evaluation of the samples followed the Avidin-Biotin-Peroxidase Complex Method as described by [8]. The lobulated mass (1.6 kg) had greenish-brown fluid-filled cysts on cut surface and presented multiple multi-layered epithelial intratubular papillary growths supported by fibrovascular connective tissue stroma with ductal ectasis containing homogenous eosinophilic materials and inter-tubular fibrosis. The mass also stained positively to vimentin and α-Smooth muscle actin but negatively to oestrogen receptor, progesterone receptor, and E-cadherin antibodies. The definitive diagnosis is a ductal carcinoma in situ with mixed carcinoma features showing the tendency to metastasize. Prompt diagnosis and appropriate management of the present case shall prevent future occurrence and the continuous investigations may serve as models for the understanding and management of human breast tumours. [ABSTRACT FROM AUTHOR]