A group of histone deacetylase (HDAC) inhibitors has been shown to suppress the growth of a variety of human tumor lines in vitro and in vivo and they are among the most promising candidates for anti-cancer therapeutic agents. We investigated the ability of scriptaid, a novel HDAC inhibitor and trichostatin A (TSA) to enhance cell killing by radiation in radioresistant SQ-20B cells derived from human head and neck squamous carcinoma. SQ-20B cells were treated with scriptaid or TSA in combination with radiation. Cell survival was determined by a colony formation assay and protein levels were examined by Western blotting. DNA double strand breaks were measured by a gamma-H2AX focus assay. Radiosensitization was observed for SQ-20B cells incubated with scriptaid at 5 microM or TSA at 0.1 microM for 24 h. Radiosensitization by scriptaid was accompanied by a prolonged retention of gamma-H2AX foci, suggesting that the enhancement of radiation cell killing by scriptaid involved inhibition of DNA double strand break repair. In addition, treatment with scriptaid suppressed expression of Ku80, but not Ku70. Scriptaid may be a useful radiosensitizer in the treatment of radioresistant human carcinomas.