The purpose of the research was to determine the activity of Tand B-cell specific protection in dogs 9–13 years old age with benign and malignant tumors of the mammary glands to assess the level of immune response to the pathological process. Assessment of the activity of cellular factors of immunity has been carried out by determining the number of T-lymphocytes and their subpopulations and B-lymphocytes in the peripheral blood of dogs in the reaction of spontaneous rosette formation with ram erythrocytes. The clinical classification and determination of the stage of breast tumors were carried out according to the TNM system. The studies have shown that the occurrence of breast tumor diseases in dogs has a significant effect on the activity of T- and B-cellular link of specific immunity mechanisms, especially the number of T-lymphocytes and their populations in animals with malignant neoplasia. In particular, in the blood of animals with malignant neoplasia of the mammary glands, the number of Tlymphocytes (common and, especially, theophylline resistant) decreases accompanied with an increase in the number of T-suppressors, which is an important criterion for predicting the course of the disease. These changes in the number of Tlymphocytes in the blood of dogs of the experimental groups have led to a decrease in the amount of immunoregulatory index, which indicates a decrease in lymphocytic activity. In the blood of dogs with malignant neoplasia, a tendency to increase the number of antigenic-dependent B-lymphocytes has been shown. Similar changes, only of a lesser extent, are also have been demonstrated in the blood of dogs with benign breast neoplasia. Thus, the results of the research showed that the development of mammary gland tumors in dogs was accompanied by an immunosuppressive effect on the specific defense mechanisms, especially the T-cell link of the immune response in animals with malignant neoplasms.