Most drug discovery efforts have focused on finding new DNA-damaging agents to kill tumor cells preferentially. An alternative approach is to find ways to increase tumor-specific killing by modifying tumor-specific responses to that damage. In this report, we ask whether cells lacking the G₁-S arrest in response to X-rays are more sensitive to X-ray damage when treated with agents that override G₂-M arrest. Mouse embryonic fibroblasts genetically matched to be (+) or (-) p53 and rat embryonic fibroblasts (+) or (-) for wild-type p53 function were irradiated with and without caffeine, a known checkpoint inhibitor. At low doses (500 µm), caffeine caused selective radiosensitization in the p53^(-) cells. At this low dose (where no effect was seen in p53⁽⁺⁾ cells), the p53^(-) cells showed a 50% reduction in the size of the G₂-M arrest. At higher doses (2 mm caffeine), where sensitization was seen in both p53⁽⁺⁾ and p53^(-) cells, the radiosensitization and the G₂-M override were more pronounced in the p53^(-) cells. The greater caffeine-induced radiosensitization in p53^(-) cells suggests that p53, already shown to control the G₁-S checkpoint, may also influence aspects of G₂-M arrest. These data indicate an opportunity for therapeutic gain by combining DNA-damaging agents with compounds that disrupt G₂-M arrest in tumors lacking functional p53.