Purpose : To determine whether the cytotoxicity produced by radiation can be increased by the spermine analog N 1, N 14-bis(ethyl)homospermine (BE-4-4-4). Methods and Materials : Two human tumor cell lines, SF-126 and U-251 MG, were either treated with 0.1 or 0.4 μM BE-4-4-4 for 3 or 4 days, or with 0.2 μM BE-4-4-4 for 4 days. At the end of BE-4-4-4 treatment, cells were irradiated and assayed immediately. Polyamine levels, cell survival, and cell number were determined. Results : In SF-126 cells, treatment with 0.2 μM BE-4-4-4 for 4 days killed about 50% of the cells and also increased the cytotoxicity of radiation. The dose enhancement ratio was ∼1.3:1.5, which is similar to that reported for α-difluoromethylornithine. Polyamine levels were partially depleted, and growth was inhibited to about 60% of control levels. Pretreatment of cells with either 0.1 or 0.4 μM BE-4-4-4 for 3 or 4 days produced less of an increase in radiation-induced cytotoxicity, even though these exposures killed 30–40% or 60–90% of the cells, respectively. Similar treatment with 0.1–0.4 μM BE-4-4-4 in U-251 MG cells had minimal effects on cytotoxicity and growth inhibition, while treatment with 1.0 μM and 2.0 μM BE-4-4-4 for 4 days produced more than a 50% depletion in polyamine levels and partial inhibition in growth, but failed to demonstrate radiopotentiation. Conclusion : The cytotoxic polyamine analog BE-4-4-4 can increase the cytotoxicity caused by radiation in at least one cell line. The amount of potentiation depends on the concentration of the analog, with the most occurring at the intermediate concentration. Because we did not observe potentiation in both cell lines, and because of the dose dependence seen in SF-126 cells, the clinical efficacy produced by combined BE-4-4-4 and radiation protocols may be limited.