The aim of this work was a study of the functional activity of neutrophils and peripheral blood monocytes in rats with the transplanted Walker carcinosarcoma as potential predictors of the sensitivity of this tumor to doxorubicin treatment. This study provides an evidence that such indices of the functional activity of circulating phagocytes of the tumor-bearing rats as the quantity and the phagocytosis intensity of monocytes, as well as the intensity of ROS production by monocytes and neutrophils, may reflect the degree of sensitivity of the tumor to doxorubicin. So it was shown that the growth of the resistant tumor caused a significant increase of the number of circulating phagocytic cells and the intensity of phagocytosis by more than 100% (p < 0.001) compared with the corresponding indices of intact rats and rats with the parental variant of the tumor. The ability of blood mono-cytes and neutrophils of rats with a resistant tumor to produce ROS was also significantly different from that in intact rats and animals with the parental carcinosarcoma variant. The predictive value of these indices is especially important in the dynamic monitoring of the development of tumor drug resistance during long-term cancer chemotherapy. Considering the standard 2 - 3 week interval between the courses of cancer therapy and the short lifetime of circulating phagocytes, an assessment of the indices of their functional activity before each subsequent course can be considered as a pretreatment assessment. Meanwhile, further studies are needed to determine the spectrum of malignant neoplasms for which the degree of tumor drug resistance correlates with the functional activity of circulating phagocytes.