ErPC sensitizes human malignant glioma cell lines to radiation-induced cell death. Induction of apoptosis and clonogenic cell survival was evaluated in U87MG, A172 and T98G cells upon irradiation (1–10 Gy) or treatment with ErPC (0–100 μM). Cell death was quantified 24–72 h after treatment by fluorescence microscopy using combined staining with Hoechst 33342 and propidium iodide. The biomathematical evaluation of putative additive or synergistic effects of the combination was performed by isobologram analysis . Analysis of combined treatment efficacy was performed with 10 Gy and 25 μM ErPC (T98G, A172), or 10 Gy and 75 μM ErPC (U87MG) 48 h after treatment. Values located within the envelope of additivity (grey region) are indicative for additive effects, values located below the envelope of additivity are indicative for synergistic increase in cytotoxicity. Combined treatment with ErPC increases cytotoxic efficacy of ionizing radiation (10 Gy, 48 h) (A, B) in a synergistic (T98G, A172 cells) or (C) additive manner (U87MG cells).