Squamous cell carcinoma of the mammary gland is rare in both veterinary and human medicine. Whereas human metaplastic and squamous variants are known, the objectives of the current study were to ascertain the presence of such entities in canine mammary tumors and to distinguish them from other (epidermal, sweat gland) squamous tumors that may develop in the same area. A panel of antibodies (anti-cytokeratin [CK] 19, CK 14, CK 5/6, pancytokeratin, and vimentin) was used on 18 mammary gland malignancies with squamous features and 16 malignant skin tumors (11 squamous cell carcinomas of the skin and 5 sweat glands). Fifteen of the 18 mammary carcinomas were classified as metaplastic carcinomas, and the remaining 3 were classified as squamous cell carcinomas. The 2 most useful markers to establish the histogenesis of mammary tumors were pancytokeratin and CK 19. All other antibodies were equally expressed (CK 14 and 5/6) in all histotypes. The antibody panel discriminated primary epidermal squamous tumors (pancytokeratin positive and CK 19 negative) from gland-derived squamous neoplasms (pancytokeratin positive and CK 19 positive) but failed to distinguish primary mammary tumors from other squamous tumors of glandular origin.