The observation that radiation treatment to a local area of the body results in an antitumor effect for tumors distant to the radiation site has been termed the "abscopal effect." To understand the mechanism of this unusual phenomenon, we examined whether the effect was mediated through p53, a protein complex up-regulated in irradiated cells. Non-tumor-bearing legs of C57BL/6 (wild-type p53) and p53 null B6.129S2-Trp53(tm1Tyj) mice were irradiated to determine whether an abscopal effect could be observed against Lewis lung carcinoma (LLC) and T241 (fibrosarcoma) implanted at a distant site. In mice with wild-type p53, both LLC and T241 tumors implanted into the midline dorsum grew at a significantly slower rate when the leg of the animal was exposed to five 10-Gy fractions of radiation compared with sham-irradiated animals, suggesting that the abscopal effect is not tumor specific. When the radiation dose to the leg was reduced (twelve fractions of 2 Gy each), the inhibition of LLC tumor growth was decreased indicating a radiation-dose dependency for the abscopal effect. In contrast, when the legs of p53 null animals or wild-type p53 mice treated with pifithrin-alpha (a p53 blocker) were irradiated (five 10-Gy fractions), tumor growth was not delayed. These data implicate p53 as a key mediator of the radiation-induced abscopal effect and suggest that pathways downstream of p53 are important in eliciting this response.