INTRODUCTION: (125)I-labeled monoclonal antibodies ((125)I-mAbs) can efficiently treat small solid tumors. Here, we investigated the role of apoptosis, autophagy and mitotic catastrophe in (125)I-mAb toxicity in p53(-/-) and p53(+/+) cancer cells. METHODS: We exposed p53(-/-) and p53(+/+) HCT116 cells to increasing activities of internalizing (cytoplasmic location) anti-HER1 (125)I-mAbs, or non-internalizing (cell surface location) anti-CEA (125)I-mAbs. For each targeting model we established the relationship between survival and mean nucleus absorbed dose using the MIRD formalism. RESULTS: In both p53(-/-) and p53(+/+) HCT116 cells, anti-CEA (125)I-mAbs were more cytotoxic per Gy than anti-HER1 (125)I-mAbs. Sensitivity to anti-CEA (125)I-mAbs was p53-independent, while sensitivity to anti-HER1 (125)I-mAbs was higher in p53(-/-) HCT 116 cells, suggesting that they act through different signaling pathways. Apoptosis was only induced in p53(+/+) HCT116 cells and could not explain cell membrane radiation sensitivity. Inhibition of autophagy did not modify the cell response to (125)I-mAbs. By contrast, mitotic death was similarly induced in both p53(-/-) and p53(+/+) HCT116 cells by the two types of (125)I-mAbs. We also showed using medium transfer experiments that γ-H2AX foci were produced in bystander cells. CONCLUSION: Cell membrane sensitivity to (125)I-mAbs is not mediated by apoptosis and is p53-independent. Bystander effects-mediated mitotic death could be involved in the efficacy of (125)I-mAbs binding cell surface receptors.