Thirty-two canine mammary tubulopapillary carcinomas and 14 simple adenomas were studied by immunohistochemistry for the expression of UDP-galactose:ceramide galactosyltransferase (UGT8). The majority of tissue specimens (57%) representing adenomas had no or weak reaction with anti-UGT8 antibodies (0-2 pts according to IRS scale) in comparison to the majority of carcinomas (90%) which stained with high intensities (3-9 pts according to IRS scale). When the average values of the reaction intensities (IRS) for malignant and benign tumors were compared, using the Mann-Whitney U-test, significant differences in UGT8 expression between them were found (P < 0.001). Mammary tubulopapillary carcinomas were further analyzed by IHC and the same rabbit polyclonal antibody directed against UGT8 according to their malignancy grade. It was found that the level of UGT8 increased in tumor specimens together with their grading. A comparison of the average values of the reaction intensity (IRS scale) revealed a significant difference (Mann-Whitney U-test, P < 0.05) in UGT8 expression between tumors representing malignancy grades G3 and G1. Based on the obtained results, it is proposed that UGT8 is associated with malignancy of canine mammary gland cells and may have a potential value as a diagnostic marker.