The aim of the study was to investigate E-cadherin expression in correlation with other neoplasm traits such as: histological type, the differentiation grade and proliferative activity. Material for the investigation comprised mammary gland tumours, collected from dogs, the patients of veterinary clinics, during surgical procedures and archival samples. All together 21 adenomas, 32 complex carcinomas, 35 simple carcinomas and 13 solid carcinomas were qualified for further investigation. E-cadherin expression was higher in adenomas as compared with carcinomas but lower in solid carcinomas as compared with simple and complex carcinomas. More over, the expression of E-cadherin decreased with the increase in the neoplasm malignancy and proliferative activity (value of the mitotic index and number of cells showing Ki67). The study has shown that the expression of E-cadherin can be used as a prognostic factor.