This article discusses one of the options for reducing the post-therapeutic complications caused by antineoplastic drugs used in the treatment of mammary glands cancer in cats. There are various ways to reduce the toxicity of chemotherapy drugs. It give an example of the use of autologous blood of illness cats, when incubating (mixing) it with doxorubicin, (the drug belongs to the group of cytostatics). This chemotherapy drug is used as monotherapy in the treatment of this disease in these animals. The doxorubicin has a pronounced cardiotoxic side effect, which can cause damage to the cells of the heart muscle (cardiomyocytes), which in turn leads to death due to the development of myocardial infarction. Below is a variant of the study, as well as a description of the results of these studies (the numerical values of the results are shown in the table), which are designed to neutralize this side effect. The described technique has not been previously used in cats. In addition, the above manipulations are quite simple and inexpensive to use. Fiat plays an important role in the choice of methods and ways of treating tumor diseases