Agents that induce DNA double-strand breaks (DSBs), such as ionizing radiation, are frequently used in cancer therapy. H2AX is rapidly phosphorylated in response to DSBs and serves as a way to measure the extent of DSBs induced in patient cells. We have previously reported a flow cytometry-based method for measuring H2AX phosphorylation in pa-tient cells undergoing radiotherapy. To be able to implement measurement of H2AX phosphorylation in clinical practice, we have characterized calibrators for the flow cytometry analysis based on phosphopeptide-coated beads and fixed cells. The calibrator beads and fixed cells lost less than 11% of the signal after storage for 40 days under optimal conditions and were able to correct for day-to-day variation in method performance.