Limited knowledge is available on myeloid derived suppressor cells (MDSCs) of rat origin. We examined the myeloid cells from peripheral blood, bone marrow and spleens of healthy and mammary tumor bearing rats employing a novel immunophenotyping strategy with CD172a, HIS48(+), and Rp-1 antibodies. We addressed rat granulocytes by Rp-1 positivity and used HIS48(+) in discrimination of two mononuclear cell subsets. An expansion of granulocyte numbers was detected in peripheral blood and spleens of mammary tumor-bearing animals. The purified granulocytes were able to impair antigen-specific helper T-cell proliferation, and therefore nominated as granulocytic MDSCs of this rat tumor model. HIS48(+) mononuclear cell numbers were also increased in the blood and spleens of mammary tumor bearing rats with a lower MHC class II positivity. Despite the lack of an antigen specific suppression of CD4(+) T cells, HIS48(+) monocytes resemble monocytic MDSCs with their inflammatory phenotype. Together, these results provide evidence for the existence and phenotypic characterization of a granulocytic MDSC subset in a rat model of mammary carcinoma. (C) 2015 Elsevier Inc. All rights reserved.