The authors present a case where the left inguinal mammary gland of a ten year old Cocker Spaniel bitch contained a complex carcinoma of 4.5 cm in largest diameter, showing sebaceous differentiation, according to histopathology. The microscopic examination revealed several islands resembling sebaceous gland morphology without cytopathological signs of malignity. The sebaceous area inside the complex carcinoma was made up of characteristic dilated ducts, sebocytes, and differentiated sebaceous gland formations. It was heavily infiltrated with lymphocytes. The differentiated sebocytes showed signs of holocrine secretion. In the lobules of the sebaceous gland basal substitute cells, intermedier cells, mature sebocytes and sebaceous ducts were also discernible. The examination was supplemented with immunohistochemical method.