In the context of high-grade gliomas such as glioblastoma (GBM), the immune part of the tumor microenvironment (TME) is involved in tumor growth and tumor recurrence. It is mostly represented by high amount of macrophages and low amount of lymphocytes. GBM in itself as well as x-ray-based radiotherapy, a standard treatment for brain tumors, are also associated with systemic effects like lymphopenia that correlates with a poor prognosis. This contributes to the immune-suppressive nature of the TME and may explain the lack of the anti-tumor immune response. Radiation-induced lymphopenia (RIL) is generally evaluated on CD4 ⁺ and CD8 ⁺ count or on a CBC (complete blood count), but the heterogeneity of the subtypes prompts us to explore them in detail to better understand the cellular response to brain irradiation. To facilitate and develop the evaluation of x-ray brain exposure on circulating immune cells, we developed a reproducible and reliable method to quantify the variation of lymphoid and myeloid subtypes using flow cytometry after brain irradiation in the rodent.