OBJECTIVE Because ribonucleotide reductase (RR) plays a role in DNA repair, it may serve as a molecular target for radiosensitization. Unlike previously investigated RR inhibitors, Triapine potently inhibits both RR holoenzymes. Therefore, the effects of Triapine on tumor cell radiosensitivity were investigated. METHODS The effects of Triapine on the in vitro radiosensitivity of three human tumor cell lines and one normal cell line were evaluated using a clonogenic assay. Growth delay was used to evaluate the effects of Triapine on in vivo tumor radiosensitivity. The levels of the RR subunits were determined using immunoblot analysis and DNA damage and repair were evaluated using gammaH2AX foci. RESULTS Exposure of the tumor cell lines to Triapine before or immediately after irradiation resulted in an increase in radiosensitivity. In contrast, Triapine enhanced the radiosensitivity of the normal fibroblast cell line only when the exposure was before irradiation. There were no consistent differences between cell lines with respect to the expression of the RR subunits. Whereas Triapine had no effect on radiation-induced gammaH2AX foci at 1 hour, the number of gammaH2AX foci per cell was significantly greater in the Triapine-treated cells at 24 hours after irradiation, suggesting the presence of unrepaired DNA damage. Triapine administration to mice bearing tumor xenografts immediately after irradiation resulted in a greater than additive increase in radiation-induced tumor growth delay. CONCLUSIONS These results indicate that Triapine can enhance tumor cell radiosensitivity in vitro and in vivo and suggest that this effect involves an inhibition of DNA repair.